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Mosses are bryophytes that live in many environments and are characterized by their short flat leaves, root-like rhizoids, and peristomes.
Learning Objectives
• Describe the distinguishing traits of mosses
Key Points
• Mosses slow down erosion, store moisture and soil nutrients, and provide shelter for small animals and food for larger herbivores.
• Mosses have green, flat structures that resemble true leaves, which absorb water and nutrients; some mosses have small branches.
• Mosses have traits that are adaptations to dry land, such as stomata present on the stems of the sporophyte.
• Mosses are anchored to the substrate by rhizoids, which originate from the base of the gametophyte.
• The moss life cycle follows the pattern of alternation of generations where gametophytes form male and female gametophores, which fertilize to form the sporophyte; spores are released from the sporophyte to produce new gametophytes.
• The concentric tissue around the mouth of the capsule is made of triangular, close-fitting units that open and close to release spores, and the peristome increases the spread of spores after the tip of the capsule falls off at dispersal.
Key Terms
• peristome: one or two rings of tooth-like appendages surrounding the opening of the capsule of many mosses that aid in spreading spores
• rhizoid: a rootlike structure that acts as support and anchors the plant to its substrate
• seta: the stalk of a moss sporangium, or occasionally in a liverwort
Mosses
More than 10,000 species of mosses have been cataloged. Their habitats vary from the tundra, where they are the main vegetation, to the understory of tropical forests. In the tundra, the mosses’ shallow rhizoids allow them to fasten to a substrate without penetrating the frozen soil. Mosses slow down erosion, store moisture and soil nutrients, and provide shelter for small animals as well as food for larger herbivores, such as the musk ox. Mosses are very sensitive to air pollution and are used to monitor air quality. They are also sensitive to copper salts. Such salts are a common ingredient of compounds marketed to eliminate mosses from lawns.
Mosses form diminutive gametophytes, which are the dominant phase of the life cycle. Green, flat structures resembling true leaves, but lacking vascular tissue are attached in a spiral to a central stalk or seta. The plants absorb water and nutrients directly through these leaf-like structures. The seta (plural, setae) contains tubular cells that transfer nutrients from the base of the sporophyte (the foot) to the sporangium. Some mosses have small branches. Some primitive traits of green algae, such as flagellated sperm, are still present in mosses that are dependent on water for reproduction. Other features of mosses are adaptations to dry land. For example, stomata are present on the stems of the sporophyte and a primitive vascular system runs up the sporophyte’s stalk. Additionally, mosses are anchored to the substrate, whether it is soil, rock, or roof tiles, by multicellular rhizoids. These structures are precursors of roots. They originate from the base of the gametophyte, but are not the major route for the absorption of water and minerals. The lack of a true root system explains why it is so easy to rip moss mats from a tree trunk.
The moss life cycle follows the pattern of alternation of generations. The most familiar structure is the haploid gametophyte, which germinates from a haploid spore and forms first a protonema: usually, a tangle of single-celled filaments that hug the ground. Cells akin to an apical meristem actively divide and give rise to a gametophore, consisting of a photosynthetic stem and foliage-like structures. Rhizoids form at the base of the gametophore. Gametangia of both sexes develop on separate gametophores. The male organ (the antheridium) produces many sperm, whereas the archegonium (the female organ) forms a single egg. At fertilization, the sperm swims down the neck to the venter and unites with the egg inside the archegonium. The zygote, protected by the archegonium, divides and grows into a sporophyte, still attached by its foot to the gametophyte.
A structure called a peristome increases the spread of spores after the tip of the capsule falls off at dispersal. The concentric tissue around the mouth of the capsule is made of triangular, close-fitting units, a little like “teeth”; these open and close depending on moisture levels, periodically releasing spores. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/29%3A_Seedless_Plants/29.02%3A_Bryophytes_Have_a_Dominant_Gametophyte_Generation/29.2C%3A_Mosses.txt |
Sporophytes (2n) undergo meiosis to produce spores that develop into gametophytes (1n) which undergo mitosis.
Learning Objectives
• Describe the role of the sporophyte and gametophyte in plant reproduction
Key Points
• The diploid stage of a plant (2n), the sporophyte, bears a sporangium, an organ that produces spores during meiosis.
• Homosporous plants produce one type of spore which develops into a gametophyte (1n) with both male and female organs.
• Heterosporous plants produce separate male and female gametophytes, which produce sperm and eggs, respectively.
• In seedless plants, male gametangia (antheridium) release sperm, which can then swim to and fertilize an egg at the female gametangia (archegonia); this mode of reproduction is replaced by pollen production in seed plants.
Key Terms
• gametophyte: a plant (or the haploid phase in its life cycle) that produces gametes by mitosis in order to produce a zygote
• gametangium: an organ or cell in which gametes are produced that is found in many multicellular protists, algae, fungi, and the gametophytes of plants
• sporopollenin: a combination of biopolymers observed in the tough outer layer of the spore and pollen wall
• syngamy: the fusion of two gametes to form a zygote
• sporophyte: a plant (or the diploid phase in its life cycle) that produces spores by meiosis in order to produce gametophytes
Sporangia in Seedless Plants
The sporophyte of seedless plants is diploid and results from syngamy (fusion) of two gametes. The sporophyte bears the sporangia (singular, sporangium): organs that first appeared in the land plants. The term “sporangia” literally means “spore in a vessel”: it is a reproductive sac that contains spores. Inside the multicellular sporangia, the diploid sporocytes, or mother cells, produce haploid spores by meiosis, where the 2n chromosome number is reduced to 1n (note that many plant sporophytes are polyploid: for example, durum wheat is tetraploid, bread wheat is hexaploid, and some ferns are 1000-ploid). The spores are later released by the sporangia and disperse in the environment.
Two different spore-forming methods are used in land plants, resulting in the separation of sexes at different points in the lifecycle. Seedless, non- vascular plants produce only one kind of spore and are called homosporous. The gametophyte phase (1n) is dominant in these plants. After germinating from a spore, the resulting gametophyte produces both male and female gametangia, usually on the same individual. In contrast, heterosporous plants produce two morphologically different types of spores. The male spores are called microspores, because of their smaller size, and develop into the male gametophyte; the comparatively larger megaspores develop into the female gametophyte. Heterospory is observed in a few seedless vascular plants and in all seed plants.
When the haploid spore germinates in a hospitable environment, it generates a multicellular gametophyte by mitosis. The gametophyte supports the zygote formed from the fusion of gametes and the resulting young sporophyte (vegetative form). The cycle then begins anew.
The spores of seedless plants are surrounded by thick cell walls containing a tough polymer known as sporopollenin. This complex substance is characterized by long chains of organic molecules related to fatty acids and carotenoids: hence the yellow color of most pollen. Sporopollenin is unusually resistant to chemical and biological degradation. In seed plants, which use pollen to transfer the male sperm to the female egg, the toughness of sporopollenin explains the existence of well-preserved pollen fossils. Sporopollenin was once thought to be an innovation of land plants; however, the green algae, Coleochaetes, also forms spores that contain sporopollenin.
Gametangia in Seedless Plants
Gametangia (singular, gametangium) are organs observed on multicellular haploid gametophytes. In the gametangia, precursor cells give rise to gametes by mitosis. The male gametangium (antheridium) releases sperm. Many seedless plants produce sperm equipped with flagella that enable them to swim in a moist environment to the archegonia: the female gametangium. The embryo develops inside the archegonium as the sporophyte. Gametangia are prominent in seedless plants, but are replaced by pollen grains in seed-producing plants. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/29%3A_Seedless_Plants/29.02%3A_Bryophytes_Have_a_Dominant_Gametophyte_Generation/29.2D%3A_Sporophytes_and_Gametophytes_in_Seedless_Plants.txt |
Seedless vascular plants, which reproduce and spread through spores, are plants that contain vascular tissue, but do not flower or seed.
Learning Objectives
• Evaluate the evolution of seedless vascular plants
Key Points
• The life cycle of seedless vascular plants alternates between a diploid sporophyte and a haploid gametophyte phase.
• Seedless vascular plants reproduce through unicellular, haploid spores instead of seeds; the lightweight spores allow for easy dispersion in the wind.
• Seedless vascular plants require water for sperm motility during reproduction and, thus, are often found in moist environments.
Key Terms
• gametophyte: a plant (or the haploid phase in its life cycle) that produces gametes by mitosis in order to produce a zygote
• sporophyte: a plant (or the diploid phase in its life cycle) that produces spores by meiosis in order to produce gametophytes
• tracheophyte: any plant possessing vascular tissue (xylem and phloem), including ferns, conifers, and flowering plants
Seedless Vascular Plants
The vascular plants, or tracheophytes, are the dominant and most conspicuous group of land plants. They contain tissue that transports water and other substances throughout the plant. More than 260,000 species of tracheophytes represent more than 90 percent of the earth’s vegetation. By the late Devonian period, plants had evolved vascular tissue, well-defined leaves, and root systems. With these advantages, plants increased in height and size and were able to spread to all habitats.
Seedless vascular plants are plants that contain vascular tissue, but do not produce flowers or seeds. In seedless vascular plants, such as ferns and horsetails, the plants reproduce using haploid, unicellular spores instead of seeds. The spores are very lightweight (unlike many seeds), which allows for their easy dispersion in the wind and for the plants to spread to new habitats. Although seedless vascular plants have evolved to spread to all types of habitats, they still depend on water during fertilization, as the sperm must swim on a layer of moisture to reach the egg. This step in reproduction explains why ferns and their relatives are more abundant in damp environments, including marshes and rainforests. The life cycle of seedless vascular plants is an alternation of generations, where the diploid sporophyte alternates with the haploid gametophyte phase. The diploid sporophyte is the dominant phase of the life cycle, while the gametophyte is an inconspicuous, but still-independent, organism. Throughout plant evolution, there is a clear reversal of roles in the dominant phase of the life cycle.
29.3B: Vascular Tissue- Xylem and Phloem
Xylem and phloem form the vascular system of plants to transport water and other substances throughout the plant.
Learning Objectives
• Describe the functions of plant vascular tissue
Key Points
• Xylem transports and stores water and water-soluble nutrients in vascular plants.
• Phloem is responsible for transporting sugars, proteins, and other organic molecules in plants.
• Vascular plants are able to grow higher than other plants due to the rigidity of xylem cells, which support the plant.
Key Terms
• xylem: a vascular tissue in land plants primarily responsible for the distribution of water and minerals taken up by the roots; also the primary component of wood
• phloem: a vascular tissue in land plants primarily responsible for the distribution of sugars and nutrients manufactured in the shoot
• tracheid: elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts
Vascular Tissue: Xylem and Phloem
The first fossils that show the presence of vascular tissue date to the Silurian period, about 430 million years ago. The simplest arrangement of conductive cells shows a pattern of xylem at the center surrounded by phloem. Together, xylem and phloem tissues form the vascular system of plants.
Xylem is the tissue responsible for supporting the plant as well as for the storage and long-distance transport of water and nutrients, including the transfer of water-soluble growth factors from the organs of synthesis to the target organs. The tissue consists of vessel elements, conducting cells, known as tracheids, and supportive filler tissue, called parenchyma. These cells are joined end-to-end to form long tubes. Vessels and tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at the ends. It is the thick walls of the tracheids that provide support for the plant and allow it to achieve impressive heights. Tall plants have a selective advantage by being able to reach unfiltered sunlight and disperse their spores or seeds further away, thus expanding their range. By growing higher than other plants, tall trees cast their shadow on shorter plants and limit competition for water and precious nutrients in the soil. The tracheids do not have end openings like the vessels do, but their ends overlap with each other, with pairs of pits present. The pit pairs allow water to pass horizontally from cell to cell.
Phloem tissue is responsible for translocation, which is the transport of soluble organic substances, for example, sugar. The substances travel along sieve elements, but other types of cells are also present: the companion cells, parenchyma cells, and fibers. The end walls, unlike vessel members in xylem, do not have large openings. The end walls, however, are full of small pores where cytoplasm extends from cell to cell. These porous connections are called sieve plates. Despite the fact that their cytoplasm is actively involved in the conduction of food materials, sieve-tube members do not have nuclei at maturity. The activity of the sieve tubes is controlled by companion cells through plasmadesmata. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/29%3A_Seedless_Plants/29.03%3A_Tracheophytes_Have_a_Dominant_Sporophyte_Generation/29.3A%3A_Seedless_Vascular_Plants.txt |
Roots support plants by anchoring them to soil, absorbing water and minerals, and storing products of photosynthesis.
Learning Objectives
• Explain how roots provide support for plants
Key Points
• There are two main types of root systems: tap root systems consist of one main root that grows down vertically with smaller lateral roots growing off of the main root, while fibrous root systems form a dense network of roots near the soil surface.
• Roots can be modified to store food or starches and to provide additional support for plants; many vegetables, such as carrots, are modified roots.
• A zone of cell division, a zone of elongation, and a zone of maturation and differentiation make up a root tip, where the root cells divide, grow, and differentiate into specialized cells.
• The vascular system of roots is surrounded by an epidermis, which regulates materials that enter the root’s vascular system.
Key Terms
• endodermis: in a plant stem or root, a cylinder of cells that separates the outer cortex from the central core and controls the flow of water and minerals within the plant
• suberin: a waxy material found in bark that can repel water
• pericycle: in a plant root, the cylinder of plant tissue between the endodermis and phloem
Roots: Support for the Plant
Roots are not well preserved in the fossil record. Nevertheless, it seems that roots appeared later in evolution than vascular tissue. The development of an extensive network of roots represented a significant new feature of vascular plants. Roots provided seed plants with three major functions: anchoring the plant to the soil, absorbing water and minerals and transporting them upwards, and storing the products of photosynthesis. Importantly, roots are modified to absorb moisture and exchange gases. In addition, while most roots are underground, some plants have adventitious roots, which emerge above the ground from the shoot.
Types of Root Systems
There are mainly two types of root systems. Dicots (flowering plants with two embryonic seed leaves) have a tap root system while monocots (flowering plants with one embryonic seed leaf) have a fibrous root system. A tap root system has a main root that grows down vertically from which many smaller lateral roots arise. Dandelions are a good example; their tap roots usually break off when trying to pull these weeds; they can regrow another shoot from the remaining root.
A tap root system penetrates deep into the soil. In contrast, a fibrous root system is located closer to the soil surface, forming a dense network of roots that also helps prevent soil erosion (lawn grasses are a good example, as are wheat, rice, and corn). In addition, some plants actually have a combination of tap root and fibrous roots. Plants that grow in dry areas often have deep root systems, whereas plants growing in areas with abundant water tend to have shallower root systems.
Root Growth and Anatomy
Root growth begins with seed germination. When the plant embryo emerges from the seed, the radicle of the embryo forms the root system. The tip of the root is protected by the root cap, a structure exclusive to roots and unlike any other plant structure. The root cap is continuously replaced because it gets damaged easily as the root pushes through soil. The root tip can be divided into three zones: a zone of cell division, a zone of elongation, and a zone of maturation and differentiation. The zone of cell division is closest to the root tip; it is made up of the actively-dividing cells of the root meristem. The zone of elongation is where the newly-formed cells increase in length, thereby lengthening the root. Beginning at the first root hair is the zone of cell maturation where the root cells begin to differentiate into special cell types. All three zones are in the first centimeter or so of the root tip.
The vascular tissue in the root is arranged in the inner portion of the root, which is called the vascular cylinder. A layer of cells, known as the endodermis, separates the vascular tissue from the ground tissue in the outer portion of the root. The endodermis is exclusive to roots, serving as a checkpoint for materials entering the root’s vascular system. A waxy substance called suberin is present on the walls of the endodermal cells. This waxy region, known as the Casparian strip, forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells. This ensures that only materials required by the root pass through the endodermis, while toxic substances and pathogens are generally excluded. The outermost cell layer of the root’s vascular tissue is the pericycle, an area that can give rise to lateral roots. In dicot roots, the xylem and phloem of the stele are arranged alternately in an X shape, whereas in monocot roots, the vascular tissue is arranged in a ring around the pith.
Root Modifications
Root structures may be modified for specific purposes. For example, some roots are bulbous and store starch. Aerial roots and prop roots are two forms of aboveground roots that provide additional support to anchor the plant. Tap roots, such as carrots, turnips, and beets, are examples of roots that are modified for food storage. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/29%3A_Seedless_Plants/29.03%3A_Tracheophytes_Have_a_Dominant_Sporophyte_Generation/29.3C%3A_The_Evolution_of_Roots_in_Seedless_Plants.txt |
Ferns, club mosses, horsetails, and whisk ferns are seedless vascular plants that reproduce with spores and are found in moist environments.
Learning Objectives
• Identify types of seedless vascular plants
Key Points
• Club mosses, which are the earliest form of seedless vascular plants, are lycophytes that contain a stem and microphylls.
• Horsetails are often found in marshes and are characterized by jointed hollow stems with whorled leaves.
• Photosynthesis occurs in the stems of whisk ferns, which lack roots and leaves.
• Most ferns have branching roots and form large compound leaves, or fronds, that perform photosynthesis and carry the reproductive organs of the plant.
Key Terms
• sorus: a cluster of sporangia associated with a fern leaf
• lycophyte: a tracheophyte subdivision of the Kingdom Plantae; the oldest extant (living) vascular plant division at around 410 million years old
• sporangia: enclosures in which spores are formed
Ferns and Other Seedless Vascular Plants
Water is required for fertilization of seedless vascular plants; most favor a moist environment. Modern-day seedless tracheophytes include lycophytes and monilophytes.
Phylum Lycopodiophyta: Club Mosses
The club mosses, or phylum Lycopodiophyta, are the earliest group of seedless vascular plants. They dominated the landscape of the Carboniferous, growing into tall trees and forming large swamp forests. Today’s club mosses are diminutive, evergreen plants consisting of a stem (which may be branched) and microphylls (leaves with a single unbranched vein). The phylum Lycopodiophyta consists of close to 1,200 species, including the quillworts (Isoetales), the club mosses (Lycopodiales), and spike mosses (Selaginellales), none of which are true mosses or bryophytes.
Lycophytes follow the pattern of alternation of generations seen in the bryophytes, except that the sporophyte is the major stage of the life cycle. The gametophytes do not depend on the sporophyte for nutrients. Some gametophytes develop underground and form mycorrhizal associations with fungi. In club mosses, the sporophyte gives rise to sporophylls arranged in strobili, cone-like structures that give the class its name. Lycophytes can be homosporous or heterosporous.
Phylum Monilophyta: Class Equisetopsida (Horsetails)
Horsetails, whisk ferns, and ferns belong to the phylum Monilophyta, with horsetails placed in the Class Equisetopsida. The single extant genus Equisetum is the survivor of a large group of plants, which produced large trees, shrubs, and vines in the swamp forests in the Carboniferous. The plants are usually found in damp environments and marshes.
The stem of a horsetail is characterized by the presence of joints or nodes, hence the old name Arthrophyta (arthro- = “joint”; -phyta = “plant”). Leaves and branches come out as whorls from the evenly-spaced joints. The needle-shaped leaves do not contribute greatly to photosynthesis, the majority of which takes place in the green stem.
Silica collects in the epidermal cells, contributing to the stiffness of horsetail plants. Underground stems known as rhizomes anchor the plants to the ground. Modern-day horsetails are homosporous and produce bisexual gametophytes.
Phylum Monilophyta: Class Psilotopsida (Whisk Ferns)
While most ferns form large leaves and branching roots, the whisk ferns, Class Psilotopsida, lack both roots and leaves, which were probably lost by reduction. Photosynthesis takes place in their green stems; small yellow knobs form at the tip of the branch stem and contain the sporangia. Whisk ferns were considered an early pterophytes. However, recent comparative DNA analysis suggests that this group may have lost both leaves and roots through evolution and is more closely related to ferns.
Phylum Monilophyta: Class Polypodiopsida (Ferns)
With their large fronds, ferns are the most-readily recognizable seedless vascular plants. More than 20,000 species of ferns live in environments ranging from tropics to temperate forests. Although some species survive in dry environments, most ferns are restricted to moist, shaded places. Ferns made their appearance in the fossil record during the Devonian period and expanded during the Carboniferous.
The dominant stage of the life cycle of a fern is the sporophyte, which typically consists of large compound leaves called fronds. Fronds fulfill a double role; they are photosynthetic organs that also carry reproductive structure. The stem may be buried underground as a rhizome from which adventitious roots grow to absorb water and nutrients from the soil, or they may grow above ground as a trunk in tree ferns. Adventitious organs are those that grow in unusual places, such as roots growing from the side of a stem. Most ferns produce the same type of spores and are, therefore, homosporous. The diploid sporophyte is the most conspicuous stage of the life cycle. On the underside of its mature fronds, sori (singular, sorus) form as small clusters where sporangia develop. Sporangia in a sorus produce spores by meiosis and release them into the air. Those that land on a suitable substrate germinate and form a heart-shaped gametophyte, which is attached to the ground by thin filamentous rhizoids. The inconspicuous gametophyte harbors both sex gametangia. Flagellated sperm are released and swim on a wet surface to where the egg is fertilized. The newly-formed zygote grows into a sporophyte that emerges from the gametophyte, growing by mitosis into the next generation sporophyte. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/29%3A_Seedless_Plants/29.04%3A_Lycophytes_Diverged_from_the_Main_Lineage_of_Vascular_Plants/29.4D%3A_Ferns_and_Other_Seedless_Vascular_Plants.txt |
Ferns, club mosses, horsetails, and whisk ferns are seedless vascular plants that reproduce with spores and are found in moist environments.
Learning Objectives
• Identify types of seedless vascular plants
Key Points
• Club mosses, which are the earliest form of seedless vascular plants, are lycophytes that contain a stem and microphylls.
• Horsetails are often found in marshes and are characterized by jointed hollow stems with whorled leaves.
• Photosynthesis occurs in the stems of whisk ferns, which lack roots and leaves.
• Most ferns have branching roots and form large compound leaves, or fronds, that perform photosynthesis and carry the reproductive organs of the plant.
Key Terms
• sorus: a cluster of sporangia associated with a fern leaf
• lycophyte: a tracheophyte subdivision of the Kingdom Plantae; the oldest extant (living) vascular plant division at around 410 million years old
• sporangia: enclosures in which spores are formed
Ferns and Other Seedless Vascular Plants
Water is required for fertilization of seedless vascular plants; most favor a moist environment. Modern-day seedless tracheophytes include lycophytes and monilophytes.
Phylum Lycopodiophyta: Club Mosses
The club mosses, or phylum Lycopodiophyta, are the earliest group of seedless vascular plants. They dominated the landscape of the Carboniferous, growing into tall trees and forming large swamp forests. Today’s club mosses are diminutive, evergreen plants consisting of a stem (which may be branched) and microphylls (leaves with a single unbranched vein). The phylum Lycopodiophyta consists of close to 1,200 species, including the quillworts (Isoetales), the club mosses (Lycopodiales), and spike mosses (Selaginellales), none of which are true mosses or bryophytes.
Lycophytes follow the pattern of alternation of generations seen in the bryophytes, except that the sporophyte is the major stage of the life cycle. The gametophytes do not depend on the sporophyte for nutrients. Some gametophytes develop underground and form mycorrhizal associations with fungi. In club mosses, the sporophyte gives rise to sporophylls arranged in strobili, cone-like structures that give the class its name. Lycophytes can be homosporous or heterosporous.
Phylum Monilophyta: Class Equisetopsida (Horsetails)
Horsetails, whisk ferns, and ferns belong to the phylum Monilophyta, with horsetails placed in the Class Equisetopsida. The single extant genus Equisetum is the survivor of a large group of plants, which produced large trees, shrubs, and vines in the swamp forests in the Carboniferous. The plants are usually found in damp environments and marshes.
The stem of a horsetail is characterized by the presence of joints or nodes, hence the old name Arthrophyta (arthro- = “joint”; -phyta = “plant”). Leaves and branches come out as whorls from the evenly-spaced joints. The needle-shaped leaves do not contribute greatly to photosynthesis, the majority of which takes place in the green stem.
Silica collects in the epidermal cells, contributing to the stiffness of horsetail plants. Underground stems known as rhizomes anchor the plants to the ground. Modern-day horsetails are homosporous and produce bisexual gametophytes.
Phylum Monilophyta: Class Psilotopsida (Whisk Ferns)
While most ferns form large leaves and branching roots, the whisk ferns, Class Psilotopsida, lack both roots and leaves, which were probably lost by reduction. Photosynthesis takes place in their green stems; small yellow knobs form at the tip of the branch stem and contain the sporangia. Whisk ferns were considered an early pterophytes. However, recent comparative DNA analysis suggests that this group may have lost both leaves and roots through evolution and is more closely related to ferns.
Phylum Monilophyta: Class Polypodiopsida (Ferns)
With their large fronds, ferns are the most-readily recognizable seedless vascular plants. More than 20,000 species of ferns live in environments ranging from tropics to temperate forests. Although some species survive in dry environments, most ferns are restricted to moist, shaded places. Ferns made their appearance in the fossil record during the Devonian period and expanded during the Carboniferous.
The dominant stage of the life cycle of a fern is the sporophyte, which typically consists of large compound leaves called fronds. Fronds fulfill a double role; they are photosynthetic organs that also carry reproductive structure. The stem may be buried underground as a rhizome from which adventitious roots grow to absorb water and nutrients from the soil, or they may grow above ground as a trunk in tree ferns. Adventitious organs are those that grow in unusual places, such as roots growing from the side of a stem. Most ferns produce the same type of spores and are, therefore, homosporous. The diploid sporophyte is the most conspicuous stage of the life cycle. On the underside of its mature fronds, sori (singular, sorus) form as small clusters where sporangia develop. Sporangia in a sorus produce spores by meiosis and release them into the air. Those that land on a suitable substrate germinate and form a heart-shaped gametophyte, which is attached to the ground by thin filamentous rhizoids. The inconspicuous gametophyte harbors both sex gametangia. Flagellated sperm are released and swim on a wet surface to where the egg is fertilized. The newly-formed zygote grows into a sporophyte that emerges from the gametophyte, growing by mitosis into the next generation sporophyte. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/29%3A_Seedless_Plants/29.05%3A_Pterophytes_Are_the_Ferns_and_Their_Relatives/29.5D%3A_Ferns_and_Other_Seedless_Vascular_Plants.txt |
• 30.1: The Evolution of Seed Plants
• 30.2: Gymnosperms - Plants with "Naked Seeds"
• 30.3: Angiosperms - The Flowering Plants
• 30.4: Seeds
Angiosperms are the flowering plants; most are terrestrial and all lack locomotion. This poses several problems. Gametes are delicate single cells. For two plants to cross fertilize, there must be a mechanism for the two gametes to reach each other safely. There must also be a mechanism to disperse their offspring far enough away from the parent so that they do not have to compete with the parent for light, water, and soil minerals. The functions of the flower solve both of these problems.
• 30.5: Fruit
Angiosperms are the flowering plants; most are terrestrial and all lack locomotion. This poses several problems. Gametes are delicate single cells. For two plants to cross fertilize, there must be a mechanism for the two gametes to reach each other safely. There must also be a mechanism to disperse their offspring far enough away from the parent so that they do not have to compete with the parent for light, water, and soil minerals. The functions of the flower solve both of these problems.
30: Seed Plants
Why are they called sunflowers?
When the plant is in the bud stage, the flower tends to track the movement of the sun across the horizon, hence the name sunflower. Flowering plants were the last group of plants to evolve. The flower contains both the male and female reproductive structures, and these plants have become tremendously successful. But these plants could not have evolved without the prior evolution of the seed. So what exactly is a seed?
Seed Plants Emerge
For reproduction, early vascular plants still needed moisture. Sperm had to swim from male to female reproductive organs for fertilization. Spores also needed some water to grow and often to disperse as well. Of course, dryness and other harsh conditions made it very difficult for tiny new offspring plants to survive. With the evolution of seeds in vascular plants, all that changed. Seed plants evolved a number of adaptations that made it possible to reproduce without water. As a result, seed plants were wildly successful. They exploded into virtually all of Earth’s habitats.
Why are seeds so adaptive on land? A seed contains an embryo and a food supply enclosed within a tough coating. An embryo is a zygote that has already started to develop and grow. Early growth and development of a plant embryo in a seed is called germination. The seed protects and nourishes the embryo and gives it a huge head start in the “race” of life. Many seeds can wait to germinate until conditions are favorable for growth. This increases the offspring’s chance of surviving even more.
Other reproductive adaptations that evolved in seed plants include ovules, pollen, pollen tubes, and pollination by animals.
• An ovule is a female reproductive structure in seed plants that contains a tiny female gametophyte. The gametophyte produces an egg cell. After the egg is fertilized by sperm, the ovule develops into a seed.
• A grain of pollen is a tiny male gametophyte enclosed in a tough capsule (see Figure below). It carries sperm to an ovule while preventing it from drying out. Pollen grains can’t swim, but they are very light, so the wind can carry them. Therefore, they can travel through air instead of water.
• Wind-blown pollen might land anywhere and be wasted. Another adaptation solved this problem. Plants evolved traits that attract specific animal pollinators. Like the bee in Figure below, a pollinator picks up pollen on its body and carries it directly to another plant of the same species. This greatly increases the chance that fertilization will occur.
• Pollen also evolved the ability to grow a tube, called a pollen tube, through which sperm could be transferred directly from the pollen grain to the egg. This allowed sperm to reach an egg without swimming through a film of water. It finally freed up plants from depending on moisture to reproduce.
Individual grains of pollen may have prickly surfaces that help them stick to pollinators such as bees. What other animals pollinate plants?
Seed Plants Diverge
The first seed plants formed seeds in cones. Cones are made up of overlapping scales, which are modified leaves (see Figure below). Male cones contain pollen, and female cones contain eggs. Seeds also develop in female cones. Modern seed plants that produce seeds in cones are called gymnosperms.
Gymnosperms produce seeds in cones (left). Each scale has a seed attached (right).
Later, seed plants called angiosperms evolved. They produce flowers, which consist of both male and female reproductive structures. The female reproductive structure in a flower includes an organ called an ovary. Eggs form in ovules inside ovaries, which also enclose and protect developing seeds after fertilization occurs. In many species of flowering plants, ovaries develop into fruits, which attract animals that disperse the seeds.
Summary
• The evolution of seeds and pollen allowed plants to reproduce on land without moisture.
• Flowering plants evolved flowers with ovaries that formed fruits. They have been the most successful plants of all.
Review
1. What is a seed?
2. Describe cones.
3. Compare and contrast gymnosperms and angiosperms, and give an example of each.
4. Which major plant adaptation—vascular tissues or seeds—do you think was more important in the evolution of plants? Choose one of the two adaptations, and write a logical argument to support your choice. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/30%3A_Seed_Plants/30.01%3A_The_Evolution_of_Seed_Plants.txt |
Gymnosperms are seed plants that have evolved cones to carry their reproductive structures.
Learning Objectives
• Discuss the type of seeds produced by gymnosperms
Key Points
• Gymnosperms produce both male and female cones, each making the gametes needed for fertilization; this makes them heterosporous.
• Megaspores made in cones develop into the female gametophytes inside the ovules of gymnosperms, while pollen grains develop from cones that produce microspores.
• Conifer sperm do not have flagella but rather move by way of a pollen tube once in contact with the ovule.
Key Terms
• ovule: the structure in a plant that develops into a seed after fertilization; the megasporangium of a seed plant with its enclosing integuments
• sporophyll: the equivalent to a leaf in ferns and mosses that bears the sporangia
• heterosporous: producing both male and female gametophytes
Characteristics of Gymnosperms
Gymnosperms are seed plants adapted to life on land; thus, they are autotrophic, photosynthetic organisms that tend to conserve water. They have a vascular system (used for the transportation of water and nutrients) that includes roots, xylem, and phloem. The name gymnosperm means “naked seed,” which is the major distinguishing factor between gymnosperms and angiosperms, the two distinct subgroups of seed plants. This term comes from the fact that the ovules and seeds of gymnosperms develop on the scales of cones rather than in enclosed chambers called ovaries.
Gymnosperms are older than angiosperms on the evolutionary scale. They are found far earlier in the fossil record than angiosperms. As will be discussed in subsequent sections, the various environmental adaptations gymnosperms have represent a step on the path to the most successful (diversity-wise) clade (monophyletic branch).
Gymnosperm Reproduction and Seeds
Gymnosperms are sporophytes (a plant with two copies of its genetic material, capable of producing spores ). Their sporangia (receptacle in which sexual spores are formed) are found on sporophylls, plated scale-like structures that together make up cones. The female gametophyte develops from the haploid (meaning one set of genetic material) spores that are contained within the sporangia. Like all seed plants, gymnosperms are heterosporous: both sexes of gametophytes develop from different types of spores produced by separate cones. One type of cone is the small pollen cone, which produces microspores that subsequently develop into pollen grains. The other type of cones, the larger “ovulate” cones, make megaspores that develop into female gametophytes called ovules. Incredibly, this whole sexual process can take three years: from the production of the two sexes of gametophytes, to bringing the gametophytes together in the process of pollination, and finally to forming mature seeds from fertilized ovules. After this process is completed, the individual sporophylls separate (the cone breaks apart) and float in the wind to a habitable place. This is concluded with germination and the formation of a seedling. Conifers have sperm that do not have flagella, but instead are conveyed to the egg via a pollen tube. It is important to note that the seeds of gymnosperms are not enclosed in their final state upon the cone.
30.2B: Life Cycle of a Conifer
Conifers are monoecious plants that produce both male and female cones, each making the necessary gametes used for fertilization.
Learning Objectives
• Describe the life cycle of a gymnosperm
Key Points
• Male cones give rise to microspores, which produce pollen grains, while female cones give rise to megaspores, which produce ovules.
• The pollen tube develops from the pollen grain to initiate fertilization; the pollen grain divides into two sperm cells by mitosis; one of the sperm cells unites with the egg cell during fertilization.
• Once the ovule is fertilized, a diploid sporophyte is produced, which gives rise to the embryo enclosed in a seed coat of tissue from the parent plant.
• Fetilization and seed development can take years; the seed that is formed is made up of three tissues: the seed coat, the gametophyte, and the embryo.
Key Terms
• megaspore: the larger spore of a heterosporous plant, typically producing a female gametophyte
• microspore: a small spore, as contrasted to the larger megaspore, which develops into male gametophytes
• monoecious: having the male (stamen) and female (carpel) reproductive organs on the same plant rather than on separate plants
Life Cycle of a Conifer
Pine trees are conifers (cone bearing) and carry both male and female sporophylls on the same mature sporophyte. Therefore, they are monoecious plants. Like all gymnosperms, pines are heterosporous, generating two different types of spores: male microspores and female megaspores. In the male cones (staminate cones), the microsporocytes give rise to pollen grains by meiosis. In the spring, large amounts of yellow pollen are released and carried by the wind. Some gametophytes will land on a female cone. Pollination is defined as the initiation of pollen tube growth. The pollen tube develops slowly as the generative cell in the pollen grain divides into two haploid sperm cells by mitosis. At fertilization, one of the sperm cells will finally unite its haploid nucleus with the haploid nucleus of an egg cell.
Female cones (ovulate cones) contain two ovules per scale. One megaspore mother cell (megasporocyte) undergoes meiosis in each ovule. Three of the four cells break down leaving only a single surviving cell which will develop into a female multicellular gametophyte. It encloses archegonia (an archegonium is a reproductive organ that contains a single large egg). Upon fertilization, the diploid egg will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant. Fertilization and seed development is a long process in pine trees: it may take up to two years after pollination. The seed that is formed contains three generations of tissues: the seed coat that originates from the sporophyte tissue, the gametophyte that will provide nutrients, and the embryo itself.
In the life cycle of a conifer, the sporophyte (2n) phase is the longest phase. The gametophytes (1n), microspores and megaspores, are reduced in size. This phase may take more than one year between pollination and fertilization while the pollen tube grows towards the megasporocyte (2n), which undergoes meiosis into megaspores. The megaspores will mature into eggs (1n).
cones moves up into upper branches where it fertilizes female cones. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/30%3A_Seed_Plants/30.02%3A_Gymnosperms_-_Plants_with_Naked_Seeds/30.2A%3A_Characteristics_of_Gymnosperms.txt |
Gymnosperms are a diverse group of plants the protect their seeds with cones and do not produce flowers or fruits.
Learning Objectives
• Give examples showing the diversity of gymnosperms
Key Points
• Gymnosperms consist of four main phyla: the Coniferophyta, Cycadophyta, Gingkophyta and Gnetophyta.
• Conifers are the dominant plant of the gymnosperms, having needle-like leaves and living in areas where the weather is cold and dry.
• Cycads live in warm climates, have large, compound leaves, and are unusual in that they are pollinated by beetles rather than wind.
• Gingko biloba is the only remaining species of the Gingkophyta and is usually resistant to pollution.
• Gnetophytes are the gymnosperms believed to be most closely related to the angiosperms because of the presence of vessel elements within their stems.
Key Terms
• tracheid: elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts
• angiosperm: a plant whose ovules are enclosed in an ovary
• conifer: a plant belonging to the conifers; a cone-bearing seed plant with vascular tissue, usually a tree
Diversity of Gymnosperms
Modern gymnosperms are classified into four phyla. The first three (the Coniferophyta, Cycadophyta, and Gingkophyta) are similar in their production of secondary cambium (cells that generate the vascular system of the trunk or stem and are partially specialized for water transportation) and their pattern of seed development. However, these three phyla are not closely related phylogenetically to each other. The fourth phylum (the Gnetophyta) are considered the closest group to angiosperms because they produce true xylem tissue.
Coniferophytes
Conifers are the dominant phylum of gymnosperms, with the most variety of species. They are typically tall trees that usually bear scale-like or needle-like leaves. Water evaporation from leaves is reduced by their thin shape and the thick cuticle. Snow slides easily off needle-shaped leaves, keeping the load light and decreasing breaking of branches. Adaptations to cold and dry weather explain the predominance of conifers at high altitudes and in cold climates. Conifers include familiar evergreen trees such as pines, spruces, firs, cedars, sequoias, and yews. A few species are deciduous, losing their leaves in fall. The European larch and the tamarack are examples of deciduous conifers. Many coniferous trees are harvested for paper pulp and timber. The wood of conifers is more primitive than the wood of angiosperms; it contains tracheids, but no vessel elements, and is, therefore, referred to as “soft wood.”
Cycads
Cycads thrive in mild climates. They are often mistaken for palms because of the shape of their large, compound leaves. Cycads bear large cones and may be pollinated by beetles rather than wind, which is unusual for a gymnosperm (). They dominated the landscape during the age of dinosaurs in the Mesozoic, but only a hundred or so species persisted to modern times. Cycads face possible extinction; several species are protected through international conventions. Because of their attractive shape, they are often used as ornamental plants in gardens in the tropics and subtropics.
Gingkophytes
The single surviving species of the gingkophytes group is the Gingko biloba. Its fan-shaped leaves, unique among seed plants because they feature a dichotomous venation pattern, turn yellow in autumn and fall from the tree. For centuries, G. biloba was cultivated by Chinese Buddhist monks in monasteries, which ensured its preservation. It is planted in public spaces because it is unusually resistant to pollution. Male and female organs are produced on separate plants. Typically, gardeners plant only male trees because the seeds produced by the female plant have an off-putting smell of rancid butter.
Gingko biloba
Gingko biloba is the only surviving species of the phylum Gingkophyta. This plate from the 1870 book Flora Japonica, Sectio Prima (Tafelband) depicts the leaves and fruit of Gingko biloba, as drawn by Philipp Franz von Siebold and Joseph Gerhard Zuccarini.
Gnetophytes
Gnetophytes are the closest relative to modern angiosperms and include three dissimilar genera of plants: Ephedra, Gnetum, and Welwitschia. Like angiosperms, they have broad leaves. In tropical and subtropical zones, gnetophytes are vines or small shrubs. Ephedra occurs in dry areas of the West Coast of the United States and Mexico. Ephedra’s small, scale-like leaves are the source of the compound ephedrine, which is used in medicine as a potent decongestant. Because ephedrine is similar to amphetamines, both in chemical structure and neurological effects, its use is restricted to prescription drugs. Like angiosperms, but unlike other gymnosperms, all gnetophytes possess vessel elements in their xylem. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/30%3A_Seed_Plants/30.02%3A_Gymnosperms_-_Plants_with_Naked_Seeds/30.2C%3A_Diversity_of_Gymnosperms.txt |
Flowers are modified leaves containing the reproductive organs of angiospems; their pollination is usually accomplished by animals or wind.
Learning Objectives
• Describe the main parts of a flower and their purposes
Key Points
• Sepals, petals, carpels, and stamens are structures found in all flowers.
• To attract pollinators, petals usually exhibit vibrant colors; however, plants that depend on wind pollination contain flowers that are small and light.
• Carpels protect the female gametophytes and megaspores.
• The stigma is the structure where pollen is deposited and is connected to the ovary through the style.
• The anther, which comprises the stamen, is the site of microspore production and their development into pollen.
Key Terms
• sepal: a part of an angiosperm, and one of the component parts of the calyx; collectively the sepals are called the calyx (plural calyces), the outermost whorl of parts that form a flower
• corolla: an outermost-but-one whorl of a flower, composed of petals, when it is not the same in appearance as the outermost whorl (the calyx); it usually comprises the petal, which may be fused
• stamen: in flowering plants, the structure in a flower that produces pollen, typically consisting of an anther and a filament
• carpel: one of the individual female reproductive organs in a flower composed of an ovary, a style, and a stigma; also known as the gynoecium
Flowers
Flowers are modified leaves, or sporophylls, organized around a central stalk. Although they vary greatly in appearance, all flowers contain the same structures: sepals, petals, carpels, and stamens. The peduncle attaches the flower to the plant. A whorl of sepals (collectively called the calyx) is located at the base of the peduncle and encloses the unopened floral bud. Sepals are usually photosynthetic organs, although there are some exceptions. For example, the corolla in lilies and tulips consists of three sepals and three petals that look virtually identical. Petals, collectively the corolla, are located inside the whorl of sepals and often display vivid colors to attract pollinators. Flowers pollinated by wind are usually small, feathery, and visually inconspicuous. Sepals and petals together form the perianth. The sexual organs (carpels and stamens) are located at the center of the flower.
Styles, stigmas, and ovules constitute the female organ: the gynoecium or carpel. Flower structure is very diverse. Carpels may be singular, multiple, or fused. Multiple fused carpels comprise a pistil. The megaspores and the female gametophytes are produced and protected by the thick tissues of the carpel. A long, thin structure called a style leads from the sticky stigma, where pollen is deposited, to the ovary, enclosed in the carpel. The ovary houses one or more ovules, each of which will develop into a seed upon fertilization. The male reproductive organs, the stamens (collectively called the androecium), surround the central carpel. Stamens are composed of a thin stalk called a filament and a sac-like structure called the anther. The filament supports the anther, where the microspores are produced by meiosis and develop into pollen grains.
30.3C: The Life Cycle of an Angiosperm
Angiosperms are seed-producing plants that generate male and female gametophytes, which allow them to carry out double fertilization.
Learning Objectives
• Explain the life cycle of an angiosperm, including cross-pollination and the ways in which it takes place
Key Points
• Microspores develop into pollen grains, which are the male gametophytes, while megaspores form an ovule that contains the female gametophytes.
• In the ovule, the megasporocyte undergoes meiosis, generating four megaspores; three small and one large; only the large megaspore survives and produces the female gametophyte (embryo sac).
• When the pollen grain reaches the stigma, it extends its pollen tube to enter the ovule and deposits two sperm cells in the embryo sac.
• The two available sperm cells allow for double fertilization to occur, which results in a diploid zygote (the future embryo) and a triploid cell (the future endosperm), which acts as a food store.
• Some species are hermaphroditic (stamens and pistils are contained on a single flower), some species are monoecious (stamens and pistils occur on separate flowers, but the same plant), and some are dioecious (staminate and pistillate flowers occur on separate plants).
Key Terms
• cotyledon: the leaf of the embryo of a seed-bearing plant; after germination it becomes the first leaves of the seedling
• heterosporous: producing both male and female gametophytes
• synergid: either of two nucleated cells at the top of the embryo sac that aid in the production of the embryo; helper cells
The Life Cycle of an Angiosperm
The adult, or sporophyte, phase is the main phase of an angiosperm’s life cycle. As with gymnosperms, angiosperms are heterosporous. Therefore, they generate microspores, which will produce pollen grains as the male gametophytes, and megaspores, which will form an ovule that contains female gametophytes. Inside the anthers’ microsporangia, male gametophytes divide by meiosis to generate haploid microspores, which, in turn, undergo mitosis and give rise to pollen grains. Each pollen grain contains two cells: one generative cell that will divide into two sperm and a second cell that will become the pollen tube cell.
The ovule, sheltered within the ovary of the carpel, contains the megasporangium protected by two layers of integuments and the ovary wall. Within each megasporangium, a megasporocyte undergoes meiosis, generating four megaspores: three small and one large. Only the large megaspore survives; it produces the female gametophyte referred to as the embryo sac. The megaspore divides three times to form an eight-cell stage. Four of these cells migrate to each pole of the embryo sac; two come to the equator and will eventually fuse to form a 2n polar nucleus. The three cells away from the egg form antipodals while the two cells closest to the egg become the synergids.
The mature embryo sac contains one egg cell, two synergids (“helper” cells), three antipodal cells, and two polar nuclei in a central cell. When a pollen grain reaches the stigma, a pollen tube extends from the grain, grows down the style, and enters through the micropyle, an opening in the integuments of the ovule. The two sperm cells are deposited in the embryo sac.
A double fertilization event then occurs. One sperm and the egg combine, forming a diploid zygote, the future embryo. The other sperm fuses with the 2n polar nuclei, forming a triploid cell that will develop into the endosperm, which is tissue that serves as a food reserve. The zygote develops into an embryo with a radicle, or small root, and one ( monocot ) or two (dicot) leaf-like organs called cotyledons. This difference in the number of embryonic leaves is the basis for the two major groups of angiosperms: the monocots and the eudicots. Seed food reserves are stored outside the embryo in the form of complex carbohydrates, lipids, or proteins. The cotyledons serve as conduits to transmit the broken-down food reserves from their storage site inside the seed to the developing embryo. The seed consists of a toughened layer of integuments forming the coat, the endosperm with food reserves, and the well-protected embryo at the center.
Some species of angiosperms are hermaphroditic (stamens and pistils are contained on a single flower), some species are monoecious (stamens and pistils occur on separate flowers, but the same plant), and some are dioecious (staminate and pistillate flowers occur on separate plants). Both anatomical and environmental barriers promote cross-pollination mediated by a physical agent (wind or water) or an animal, such as an insect or bird. Cross-pollination increases genetic diversity in a species. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/30%3A_Seed_Plants/30.03%3A_Angiosperms_-_The_Flowering_Plants/30.3A%3A_Angiosperm_Flowers.txt |
Angiosperm diversity is divided into two main groups, monocot and dicots, based primarily on the number of cotyledons they possess.
Learning Objectives
• Explain how angiosperm diversity is classified
Key Points
• Angiosperm are flowering plants that are classified based on characteristics that include (but are not limited to) cotyledon structure, pollen grains, as well as flower and vascular tissue arrangement.
• Basal angiosperms, classified separately, contain features found in both monocots and dicots, as they are believed to have originated before the separation of these two main groups.
• Monocots contain a single cotyledon and have veins that run parallel to the length of their leaves; their flowers are arranged in three to six-fold symmetry.
• Dicots have flowers arranged in whorls, two cotyledons, and a vein arrangement that forms networks within their leaves.
• Monocots do not contain any true woody tissue while dicots can be herbacious or woody and have vascular tissue that forms a ring in the stem.
Key Terms
• dicot: a plant whose seedlings have two cotyledons; a dicotyledon
• angiosperm: a plant whose ovules are enclosed in an ovary
• monocot: one of two major groups of flowering plants (or angiosperms) that are traditionally recognized; seedlings typically have one cotyledon (seed-leaf)
• cotyledon: the leaf of the embryo of a seed-bearing plant; after germination it becomes the first leaves of the seedling
• basal angiosperm: the first flowering plants to diverge from the ancestral angiosperm, including a single species of shrub from New Caledonia, water lilies and some other aquatic plants, and woody aromatic plants
Diversity of Angiosperms
Angiosperms are classified in a single phylum: the Anthophyta. Modern angiosperms appear to be a monophyletic group, which means that they originated from a single ancestor. Flowering plants are divided into two major groups according to the structure of the cotyledons and pollen grains, among others. Monocots include grasses and lilies while eudicots or dicots form a polyphyletic group. However, many species exhibit characteristics that belong to either group; as such, the classification of a plant as a monocot or a eudicot is not always clearly evident. Basal angiosperms are a group of plants that are believed to have branched off before the separation into monocots and eudicots because they exhibit traits from both groups. They are categorized separately in many classification schemes. The Magnoliidae (magnolia trees, laurels, and water lilies) and the Piperaceae (peppers) belong to the basal angiosperm group.
Basal Angiosperms
Examples of basal angiosperms include the Magnoliidae, Laurales, Nymphaeales, and the Piperales. Members in these groups all share traits from both monocot and dicot groups. The Magnoliidae are represented by the magnolias: tall trees bearing large, fragrant flowers that have many parts and are considered archaic. Laurel trees produce fragrant leaves and small, inconspicuous flowers. The Laurales grow mostly in warmer climates and are small trees and shrubs. Familiar plants in this group include the bay laurel, cinnamon, spice bush, and avocado tree. The Nymphaeales are comprised of the water lilies, lotus, and similar plants; all species thrive in freshwater biomes and have leaves that float on the water surface or grow underwater. Water lilies are particularly prized by gardeners and have graced ponds and pools for thousands of years. The Piperales are a group of herbs, shrubs, and small trees that grow in the tropical climates. They have small flowers without petals that are tightly arranged in long spikes. Many species are the source of prized fragrance or spices; for example, the berries of Piper nigrum are the familiar black peppercorns that are used to flavor many dishes.
Monocots
Plants in the monocot group are primarily identified as such by the presence of a single cotyledon in the seedling. Other anatomical features shared by monocots include veins that run parallel to the length of the leaves and flower parts that are arranged in a three- or six-fold symmetry. True woody tissue is rarely found in monocots. In palm trees, vascular and parenchyma tissues produced by the primary and secondary thickening of meristems form the trunk. The pollen from the first angiosperms was monosulcate, containing a single furrow or pore through the outer layer. This feature is still seen in the modern monocots. Vascular tissue of the stem is not arranged in any particular pattern. The root system is mostly adventitious and unusually positioned, with no major tap root. The monocots include familiar plants such as the true lilies (which are the origin of their alternate name: Liliopsida), orchids, grasses, and palms. Many important crops are monocots, such as rice and other cereals, corn, sugar cane, and tropical fruits like bananas and pineapples.
Eudicots
Eudicots, or true dicots, are characterized by the presence of two cotyledons in the developing shoot. Veins form a network in leaves, while flower parts come in four, five, or many whorls. Vascular tissue forms a ring in the stem whereas in monocots, vascular tissue is scattered in the stem. Eudicots can be herbaceous (like grasses), or produce woody tissues. Most eudicots produce pollen that is trisulcate or triporate, with three furrows or pores. The root system is usually anchored by one main root developed from the embryonic radicle. Eudicots comprise two-thirds of all flowering plants. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/30%3A_Seed_Plants/30.03%3A_Angiosperms_-_The_Flowering_Plants/30.3D%3A_Diversity_of_Angiosperms.txt |
Angiosperms are the flowering plants (today the most abundant and diverse plants on earth). Most are terrestrial and all lack locomotion. This poses several problems. Gametes are delicate single cells. For two plants to cross fertilize, there must be a mechanism for the two gametes to reach each other safely. Moreover, there must also be a mechanism to disperse their offspring far enough away from the parent so that they do not have to compete with the parent for light, water, and soil minerals. The functions of the flower solve both of these problems.
The Flower and Its Pollination
In angiosperms, meiosis in the sporophyte generation produces two kinds of spores: (1) microspores which develop in the microsporangium and will germinate and develop into the male gametophyte generation and (2) megaspores that develop in the megasporangium will develop into the female gametophyte generation. Both types of sporangia are formed in flowers. In most angiosperms, the flowers are perfect: each has both microsporangia and megasporangia, although some angiosperms are imperfect, having either microsporangia or megasporangia but not both.
• Monoecious plants have both types of imperfect flower on the same plant.
• Dioecious plants have imperfect flowers on separate plants; that is, some plants are male, some female. Examples include willows, poplars, and the date palm. Most dioecious plants use an X-Y system of of sex determination like that in mammals. However, a few species use an X-to-autosome ratio system like that of Drosophila, and a very few use a ZW system like that of birds and lepidoptera.
Flowers develop from flower buds. Each bud contains 4 concentric whorls of tissue. From the outer to the inner, these develop into
• a whorl of sepals (collectively called the calyx)
• a whorl of petals (collectively called the corolla)
• stamens in which the microsporangia form
• carpels in which the megasporangia form.
Stamens
Each stamen consists of a lobed anther, containing the microsporangia and supported by a thin filament. Meiosis of the diploid microspore mother cells in the anther produces four haploid microspores. Each of these develops into a pollen grain consisting of a larger vegetative cell (also called the tube cell) inside of which is a a smaller germ cell (also called the generative cell). At some point, depending on the species, the germ cell divides by mitosis to produce 2 sperm cells.
Carpels
Carpels consist of a stigma, usually mounted at the tip of a style with an ovary at the base. Often the entire whorl of carpels is fused into a single pistil. The megasporangia, called ovules, develop within the ovary. Meiosis of the megaspore mother cell in each ovule produces 4 haploid cells, a large megaspore and 3 small cells that disintegrate.
Development of the megaspore
The nucleus of the megaspore undergoes three successive mitotic divisions. The 8 nuclei that result are distributed and partitioned off by cell walls to form the embryo sac. This is the mature female gametophyte generation. The egg cell will start the new sporophyte generation if it is fertilized. It is flanked by 2 synergid cells. In several (perhaps all) angiosperms, they secrete an attractant that guides the pollen tube through the micropyle into the embryo sac. The large central cell, which in most angiosperms contains 2 polar nuclei, will after its fertilization develop into the endosperm of the seed. It also contains 3 antipodal cells.
Pollination
When a pollen grain reaches the stigma, it germinates into a pollen tube. If it hasn't done so already, the germ cell divides by mitosis forming 2 sperm cells. These, along with the tube nucleus (also known as the vegetative nucleus), migrate down the pollen tube as it grows through the style, the micropyle, and into the ovule chamber.
In Arabidopsis the pollen tube follows a gradient of increasing concentration of a small defensin-like protein secreted by the synergids. The pollen tube with its contents makes up the mature male gametophyte generation.
Double fertilization
The pollen tube enters the ovule through the micropyle and ruptures. One sperm cell fuses with the egg forming the diploid zygote. The other sperm cell fuses with the polar nuclei forming the endosperm nucleus. Most angiosperms have two polar nuclei so the endosperm is triploid (3n). The tube nucleus disintegrates.
Most angiosperms have mechanisms by which they avoid self-fertilization.
Seeds
After double fertilization, each ovule develops into a seed, which consists of
• a plumule, made up of two embryonic leaves, which will become the first true leaves of the seedling, and a terminal (apical) bud. The terminal bud contains the meristem at which later growth of the stem takes place.
• One or two cotyledons which store food that will be used by the germinating seedling. Angiosperms that produce seeds with two cotyledons are called dicots. Examples include beans, squashes, Arabidopsis. Angiosperms whose seeds contain only a single cotyledon are monocots. Examples include corn and other grasses.
• The hypocotyl and radicle, which will grow into the part of the stem below the first node ("hypocotyl" = below the cotyledons) and primary root respectively. The development of each of the parts of the plant embryo depends on gradients of the plant hormone, auxin.
• In addition to the embryo plant (derived from the zygote), each seed is covered with protective seed coats derived from the walls of the ovule.
The food in the cotyledons is derived from the endosperm which, in turn, received it from the parent sporophyte. In many angiosperms (e.g., beans), when the seeds are mature, the endosperm has been totally consumed and its food transferred to the cotyledons. In others (some dicots and all monocots), the endosperm persists in the mature seed.
The seed is thus a dormant embryo sporophyte with stored food and protective coats. Its two functions are
• dispersal of the species to new locations (aided in angiosperms by the fruit)
• survival of the species during unfavorable climatic periods (e.g., winter). "Annual" plants (e.g., beans, cereal grains, many weeds) can survive freezing only as seeds. When the parents die in the fall, the seeds remain alive — though dormant— over the winter. When conditions are once more favorable, germination occurs and a new generation of plants develops.
Fruits
Fruits are a development of the ovary wall and sometimes other flower parts as well. As seeds mature, they release the hormone auxin, which stimulates the wall of the ovary to develop into the fruit. In fact, commercial fruit growers may stimulate fruit development in unpollinated flowers by applying synthetic auxin to the flower.
Fruits promote the dispersal of their content of seeds in a variety of ways.
• Wind. The maple "key" and dandelion parachute are examples.
• Water. Many aquatic angiosperms and shore dwellers (e.g., the coconut palm) have floating fruits that are carried by water currents to new locations.
• Hitchhikers. The cocklebur and sticktights achieve dispersal of their seeds by sticking to the coat (or clothing) of a passing animal.
• Edible fruits. Nuts and berries entice animals to eat them. Buried and forgotten (nuts) or passing through their g.i. tract unharmed (berries), the seeds may end up some distance away from the parent plant.
• Mechanical. Some fruits, as they dry, open explosively expelling their seeds. The pods of many legumes (e.g., wisteria) do this.
30.04: Seeds
The evolution of seeds allowed plants to reproduce independently of water; pollen allows them to disperse their gametes great distances.
Learning Objectives
• Recognize the significance of seed plant evolution
Key Points
• Plants are used for food, textiles, medicines, building materials, and many other products that are important to humans.
• The evolution of seeds allowed plants to decrease their dependency upon water for reproduction.
• Seeds contain an embryo that can remain dormant until conditions are favorable when it grows into a diploid sporophyte.
• Seeds are transported by the wind, water, or by animals to encourage reproduction and reduce competition with the parent plant.
Key Terms
• seed: a fertilized ovule, containing an embryonic plant
• sporophyte: a plant (or the diploid phase in its life cycle) that produces spores by meiosis in order to produce gametophytes
• pollen: microspores produced in the anthers of flowering plants
Evolution of Seed Plants
The lush palms on tropical shorelines do not depend upon water for the dispersal of their pollen, fertilization, or the survival of the zygote, unlike mosses, liverworts, and ferns of the terrain. Seed plants, such as palms, have broken free from the need to rely on water for their reproductive needs. They play an integral role in all aspects of life on the planet, shaping the physical terrain, influencing the climate, and maintaining life as we know it. For millennia, human societies have depended upon seed plants for nutrition and medicinal compounds; and more recently, for industrial by-products, such as timber and paper, dyes, and textiles. Palms provide materials including rattans, oils, and dates. Wheat is grown to feed both human and animal populations. The fruit of the cotton boll flower is harvested as a boll, with its fibers transformed into clothing or pulp for paper. The showy opium poppy is valued both as an ornamental flower and as a source of potent opiate compounds.
Seeds and Pollen as an Evolutionary Adaptation to Dry Land
Unlike bryophyte and fern spores (which are haploid cells dependent on moisture for rapid development of gametophytes ), seeds contain a diploid embryo that will germinate into a sporophyte. Storage tissue to sustain growth and a protective coat give seeds their superior evolutionary advantage. Several layers of hardened tissue prevent desiccation, freeing reproduction from the need for a constant supply of water. Furthermore, seeds remain in a state of dormancy induced by desiccation and the hormone abscisic acid until conditions for growth become favorable. Whether blown by the wind, floating on water, or carried away by animals, seeds are scattered in an expanding geographic range, thus avoiding competition with the parent plant.
Pollen grains are male gametophytes carried by wind, water, or a pollinator. The whole structure is protected from desiccation and can reach the female organs without dependence on water. Male gametes reach female gametophyte and the egg cell gamete though a pollen tube: an extension of a cell within the pollen grain. The sperm of modern gymnosperms lack flagella, but in cycads and the Gingko, the sperm still possess flagella that allow them to swim down the pollen tube to the female gamete; however, they are enclosed in a pollen grain. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/30%3A_Seed_Plants/30.04%3A_Seeds/30.4A%3A_The_Evolution_of_Seed_Plants_and_Adaptations_for_Land.txt |
Angiosperms are the flowering plants (today the most abundant and diverse plants on earth). Most are terrestrial and all lack locomotion. This poses several problems. Gametes are delicate single cells. For two plants to cross fertilize, there must be a mechanism for the two gametes to reach each other safely. Moreover, there must also be a mechanism to disperse their offspring far enough away from the parent so that they do not have to compete with the parent for light, water, and soil minerals. The functions of the flower solve both of these problems.
The Flower and Its Pollination
In angiosperms, meiosis in the sporophyte generation produces two kinds of spores: (1) microspores which develop in the microsporangium and will germinate and develop into the male gametophyte generation and (2) megaspores that develop in the megasporangium will develop into the female gametophyte generation. Both types of sporangia are formed in flowers. In most angiosperms, the flowers are perfect: each has both microsporangia and megasporangia, although some angiosperms are imperfect, having either microsporangia or megasporangia but not both.
• Monoecious plants have both types of imperfect flower on the same plant.
• Dioecious plants have imperfect flowers on separate plants; that is, some plants are male, some female. Examples include willows, poplars, and the date palm. Most dioecious plants use an X-Y system of of sex determination like that in mammals. However, a few species use an X-to-autosome ratio system like that of Drosophila, and a very few use a ZW system like that of birds and lepidoptera.
Flowers develop from flower buds. Each bud contains 4 concentric whorls of tissue. From the outer to the inner, these develop into
• a whorl of sepals (collectively called the calyx)
• a whorl of petals (collectively called the corolla)
• stamens in which the microsporangia form
• carpels in which the megasporangia form.
Stamens
Each stamen consists of a lobed anther, containing the microsporangia and supported by a thin filament. Meiosis of the diploid microspore mother cells in the anther produces four haploid microspores. Each of these develops into a pollen grain consisting of a larger vegetative cell (also called the tube cell) inside of which is a a smaller germ cell (also called the generative cell). At some point, depending on the species, the germ cell divides by mitosis to produce 2 sperm cells.
Carpels
Carpels consist of a stigma, usually mounted at the tip of a style with an ovary at the base. Often the entire whorl of carpels is fused into a single pistil. The megasporangia, called ovules, develop within the ovary. Meiosis of the megaspore mother cell in each ovule produces 4 haploid cells, a large megaspore and 3 small cells that disintegrate.
Development of the megaspore
The nucleus of the megaspore undergoes three successive mitotic divisions. The 8 nuclei that result are distributed and partitioned off by cell walls to form the embryo sac. This is the mature female gametophyte generation. The egg cell will start the new sporophyte generation if it is fertilized. It is flanked by 2 synergid cells. In several (perhaps all) angiosperms, they secrete an attractant that guides the pollen tube through the micropyle into the embryo sac. The large central cell, which in most angiosperms contains 2 polar nuclei, will after its fertilization develop into the endosperm of the seed. It also contains 3 antipodal cells.
Pollination
When a pollen grain reaches the stigma, it germinates into a pollen tube. If it hasn't done so already, the germ cell divides by mitosis forming 2 sperm cells. These, along with the tube nucleus (also known as the vegetative nucleus), migrate down the pollen tube as it grows through the style, the micropyle, and into the ovule chamber.
In Arabidopsis the pollen tube follows a gradient of increasing concentration of a small defensin-like protein secreted by the synergids. The pollen tube with its contents makes up the mature male gametophyte generation.
Double fertilization
The pollen tube enters the ovule through the micropyle and ruptures. One sperm cell fuses with the egg forming the diploid zygote. The other sperm cell fuses with the polar nuclei forming the endosperm nucleus. Most angiosperms have two polar nuclei so the endosperm is triploid (3n). The tube nucleus disintegrates.
Most angiosperms have mechanisms by which they avoid self-fertilization.
Seeds
After double fertilization, each ovule develops into a seed, which consists of
• a plumule, made up of two embryonic leaves, which will become the first true leaves of the seedling, and a terminal (apical) bud. The terminal bud contains the meristem at which later growth of the stem takes place.
• One or two cotyledons which store food that will be used by the germinating seedling. Angiosperms that produce seeds with two cotyledons are called dicots. Examples include beans, squashes, Arabidopsis. Angiosperms whose seeds contain only a single cotyledon are monocots. Examples include corn and other grasses.
• The hypocotyl and radicle, which will grow into the part of the stem below the first node ("hypocotyl" = below the cotyledons) and primary root respectively. The development of each of the parts of the plant embryo depends on gradients of the plant hormone, auxin.
• In addition to the embryo plant (derived from the zygote), each seed is covered with protective seed coats derived from the walls of the ovule.
The food in the cotyledons is derived from the endosperm which, in turn, received it from the parent sporophyte. In many angiosperms (e.g., beans), when the seeds are mature, the endosperm has been totally consumed and its food transferred to the cotyledons. In others (some dicots and all monocots), the endosperm persists in the mature seed.
The seed is thus a dormant embryo sporophyte with stored food and protective coats. Its two functions are
• dispersal of the species to new locations (aided in angiosperms by the fruit)
• survival of the species during unfavorable climatic periods (e.g., winter). "Annual" plants (e.g., beans, cereal grains, many weeds) can survive freezing only as seeds. When the parents die in the fall, the seeds remain alive — though dormant— over the winter. When conditions are once more favorable, germination occurs and a new generation of plants develops.
Fruits
Fruits are a development of the ovary wall and sometimes other flower parts as well. As seeds mature, they release the hormone auxin, which stimulates the wall of the ovary to develop into the fruit. In fact, commercial fruit growers may stimulate fruit development in unpollinated flowers by applying synthetic auxin to the flower.
Fruits promote the dispersal of their content of seeds in a variety of ways.
• Wind. The maple "key" and dandelion parachute are examples.
• Water. Many aquatic angiosperms and shore dwellers (e.g., the coconut palm) have floating fruits that are carried by water currents to new locations.
• Hitchhikers. The cocklebur and sticktights achieve dispersal of their seeds by sticking to the coat (or clothing) of a passing animal.
• Edible fruits. Nuts and berries entice animals to eat them. Buried and forgotten (nuts) or passing through their g.i. tract unharmed (berries), the seeds may end up some distance away from the parent plant.
• Mechanical. Some fruits, as they dry, open explosively expelling their seeds. The pods of many legumes (e.g., wisteria) do this.
30.05: Fruit
A fertilized, fully grown, and ripened ovary containing a seed forms what we know as fruit, important seed dispersal agents for plants.
Learning Objectives
• Recall the evolutionary advantage of fruits
Key Points
• Scientists classify fruit in many different categories that include descriptions, such as mature, fleshy, and dry; only a few are actually classified as being fleshy and sweet.
• Some fruit are developed from ovaries, while others develop from the pericarp, from clusters of flowers, or from separate ovaries in a single flower.
• Fruit are vital dispersal agents for plants; their unique shapes and features evolved to take advantage of specific dispersal modes.
• Dispersal methods of seeds within fruit include wind, water, herbivores, and animal fur.
Key Terms
• fruit: the seed-bearing part of a plant, often edible, colorful, and fragrant, produced from a floral ovary after fertilization
• pericarp: the outermost layer, or skin, of a ripe fruit or ovary
• hypanthium: the bowl-shaped part of a flower on which the sepals, petals, and stamens are borne
Fruit
In botany, a fertilized, fully-grown, and ripened ovary is a fruit. As the seed develops, the walls of the ovary in which it forms thicken and form the fruit, enlarging as the seeds grow. Many foods commonly-called vegetables are actually fruit. Eggplants, zucchini, string beans, and bell peppers are all technically fruit because they contain seeds and are derived from the thick ovary tissue. Acorns are nuts and winged, maple whirligigs (whose botanical name is samara) are also fruit. Botanists classify fruit into more than two dozen different categories, only a few of which are actually fleshy and sweet.
Mature fruit can be fleshy or dry. Fleshy fruit include the familiar berries, peaches, apples, grapes, and tomatoes. Rice, wheat, and nuts are examples of dry fruit. Another distinction is that not all fruits are derived from the ovary. For instance, strawberries are derived from the receptacle, while apples are derived from the pericarp, or hypanthium. Some fruits are derived from separate ovaries in a single flower, such as the raspberry. Other fruits, such as the pineapple, form from clusters of flowers. Additionally, some fruits, like watermelon and oranges, have rinds.
Regardless of how they are formed, fruits are an agent of seed dispersal. The variety of shapes and characteristics reflect the mode of dispersal, whether it be wind, water, or animals. Wind carries the light dry fruit of trees and dandelions. Water transports floating coconuts. Some fruits attract herbivores with color or perfume, or as food. Once eaten, tough, undigested seeds are dispersed through the herbivore’s feces. Other fruits have burs and hooks to cling to fur and hitch rides on animals. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/30%3A_Seed_Plants/30.05%3A_Fruit/30.5B%3A_Angsiosperm_Fruit.txt |
What type of fungus is this?
Obviously a mold. But what type of mold? There are thousands of known species of molds. How are they classified?
Classification of Fungi
For a long time, scientists considered fungi to be members of the plant kingdom because they have obvious similarities with plants. Both fungi and plants are immobile, have cell walls, and grow in soil. Some fungi, such as lichens, even look like plants (see Figure below).
Moss (Plant) and Lichen Growing on Tree Bark. Both fungi and moss are growing on this tree. Can you tell them apart?
The Kingdom Fungi
Today, fungi are no longer classified as plants. We now know that they have unique physical, chemical, and genetic traits that set them apart from plants and other eukaryotes. For example, the cell walls of fungi are made of chitin, not cellulose. Also, fungi absorb nutrients from other organisms, whereas plants make their own food. These are just a few of the reasons fungi are now placed in their own kingdom.
Fungal Phyla
Classification of fungi below the level of the kingdom is controversial. There is no single, widely-accepted system of fungal classification. Most classifications include several phyla (the next major taxon below the kingdom). Three of the most common phyla are compared inTable below.
Phylum Description Example
Zygomycota mainly terrestrial, live in soil and compost and on foods such as bread
black bread mold
Basidiomycota have many different shapes, considerable variation exists even within species
button mushrooms
Ascomycota found in all terrestrial ecosystems world-wide, even in Antarctica, often involved in symbiotic relationships
baker’s yeast
Summary
• Fungi used to be classified as plants. Now, they are known to have unique traits that set them apart from plants. For example, fungal cell walls contain chitin, not cellulose, and fungi absorb food rather than make their own.
• Below the level of the kingdom, classification of fungi is controversial.
Review
1. State why fungi were once classified as plants.
2. Explain the significance of the chitin cell wall of fungi.
3. Mushrooms belong to what phylum of fungi?
31.02: Fungal Forms Nutrition and Reproduction
The word fungus comes from the Latin word for mushroom. Indeed, the familiar mushrooms are fungi, but there are many other types of fungi as well (Figure \(1\)). The kingdom Fungi includes an enormous variety of living organisms collectively referred to as Eumycota, or true fungi. While scientists have identified about 100,000 species of fungi, this is only a fraction of the over 1 million species likely present on Earth. Edible mushrooms, yeasts, black mold, and Penicillium notatum (the producer of the antibiotic penicillin) are all members of the kingdom Fungi, which belongs to the domain Eukarya. As eukaryotes, a typical fungal cell contains a true nucleus and many membrane-bound organelles.
Fungi were once considered plant-like organisms; however, DNA comparisons have shown that fungi are more closely related to animals than plants. Fungi are not capable of photosynthesis: They use complex organic compounds as sources of energy and carbon. Some fungal organisms multiply only asexually, whereas others undergo both asexual reproduction and sexual reproduction. Most fungi produce a large number of spores that are disseminated by the wind. Like bacteria, fungi play an essential role in ecosystems, because they are decomposers and participate in the cycling of nutrients by breaking down organic materials into simple molecules.
Fungi often interact with other organisms, forming mutually beneficial or mutualistic associations. Fungi also cause serious infections in plants and animals. For example, Dutch elm disease is a particularly devastating fungal infection that destroys many native species of elm (Ulmus spp.). The fungus infects the vascular system of the tree. It was accidentally introduced to North America in the 1900s and decimated elm trees across the continent. Dutch elm disease is caused by the fungus Ophiostoma ulmi. The elm bark beetle acts as a vector and transmits the disease from tree to tree. Many European and Asiatic elms are less susceptible than American elms.
In humans, fungal infections are generally considered challenging to treat because, unlike bacteria, they do not respond to traditional antibiotic therapy since they are also eukaryotes. These infections may prove deadly for individuals with a compromised immune system.
Fungi have many commercial applications. The food industry uses yeasts in baking, brewing, and wine making. Many industrial compounds are byproducts of fungal fermentation. Fungi are the source of many commercial enzymes and antibiotics.
Cell Structure and Function
Fungi are eukaryotes and as such have a complex cellular organization. As eukaryotes, fungal cells contain a membrane-bound nucleus. A few types of fungi have structures comparable to the plasmids (loops of DNA) seen in bacteria. Fungal cells also contain mitochondria and a complex system of internal membranes, including the endoplasmic reticulum and Golgi apparatus.
Fungal cells do not have chloroplasts. Although the photosynthetic pigment chlorophyll is absent, many fungi display bright colors, ranging from red to green to black. The poisonous Amanita muscaria (fly agaric) is recognizable by its bright red cap with white patches (Figure \(2\)). Pigments in fungi are associated with the cell wall and play a protective role against ultraviolet radiation. Some pigments are toxic.
Like plant cells, fungal cells are surrounded by a thick cell wall; however, the rigid layers contain the complex polysaccharides chitin and glucan and not cellulose that is used by plants. Chitin, also found in the exoskeleton of insects, gives structural strength to the cell walls of fungi. The cell wall protects the cell from desiccation and predators. Fungi have plasma membranes similar to other eukaryotes, except that the structure is stabilized by ergosterol, a steroid molecule that functions like the cholesterol found in animal cell membranes. Most members of the kingdom Fungi are nonmotile. Flagella are produced only by the gametes in the primitive division Chytridiomycota.
Growth and Reproduction
The vegetative body of a fungus is called a thallus and can be unicellular or multicellular. Some fungi are dimorphic because they can go from being unicellular to multicellular depending on environmental conditions. Unicellular fungi are generally referred to as yeasts.Saccharomyces cerevisiae (baker’s yeast) and Candida species (the agents of thrush, a common fungal infection) are examples of unicellular fungi.
Most fungi are multicellular organisms. They display two distinct morphological stages: vegetative and reproductive. The vegetative stage is characterized by a tangle of slender thread-like structures called hyphae (singular, hypha), whereas the reproductive stage can be more conspicuous. A mass of hyphae is called a mycelium (Figure \(3\)). It can grow on a surface, in soil or decaying material, in a liquid, or even in or on living tissue. Although individual hypha must be observed under a microscope, the mycelium of a fungus can be very large with some species truly being “the fungus humongous.” The giant Armillaria ostoyae (honey mushroom) is considered the largest organism on Earth, spreading across over 2,000 acres of underground soil in eastern Oregon; it is estimated to be at least 2,400 years old.
Most fungal hyphae are divided into separate cells by end walls called septa (singular, septum). In most divisions (like plants, fungal phyla are called divisions by tradition) of fungi, tiny holes in the septa allow for the rapid flow of nutrients and small molecules from cell to cell along the hyphae. They are described as perforated septa. The hyphae in bread molds (which belong to the division Zygomycota) are not separated by septa. They are formed of large cells containing many nuclei, an arrangement described as coenocytic hyphae.
Fungi thrive in environments that are moist and slightly acidic, and can grow with or without light. They vary in their oxygen requirements. Most fungi are obligate aerobes, requiring oxygen to survive. Other species, such as the Chytridiomycota that reside in the rumen of cattle, are obligate anaerobes, meaning that they cannot grow and reproduce in an environment with oxygen. Yeasts are intermediate: They grow best in the presence of oxygen but can use fermentation in the absence of oxygen. The alcohol produced from yeast fermentation is used in wine and beer production, and the carbon dioxide they produce carbonates beer and sparkling wine, and makes bread rise.
Fungi can reproduce sexually or asexually. In both sexual and asexual reproduction, fungi produce spores that disperse from the parent organism by either floating in the wind or hitching a ride on an animal. Fungal spores are smaller and lighter than plant seeds, but they are not usually released as high in the air. The giant puffball mushroom bursts open and releases trillions of spores: The huge number of spores released increases the likelihood of spores landing in an environment that will support growth (Figure \(4\)).
How Fungi Obtain Nutrition
Like animals, fungi are heterotrophs: They use complex organic compounds as a source of carbon rather than fixing carbon dioxide from the atmosphere, as some bacteria and most plants do. In addition, fungi do not fix nitrogen from the atmosphere. Like animals, they must obtain it from their diet. However, unlike most animals that ingest food and then digest it internally in specialized organs, fungi perform these steps in the reverse order. Digestion precedes ingestion. First, exoenzymes, enzymes that catalyze reactions on compounds outside of the cell, are transported out of the hyphae where they break down nutrients in the environment. Then, the smaller molecules produced by the external digestion are absorbed through the large surface areas of the mycelium. As with animal cells, the fungal storage polysaccharide is glycogen rather than starch, as found in plants.
Fungi are mostly saprobes, organisms that derive nutrients from decaying organic matter. They obtain their nutrients from dead or decomposing organic matter, mainly plant material. Fungal exoenzymes are able to break down insoluble polysaccharides, such as the cellulose and lignin of dead wood, into readily absorbable glucose molecules. Decomposers are important components of ecosystems, because they return nutrients locked in dead bodies to a form that is usable for other organisms. This role is discussed in more detail later. Because of their varied metabolic pathways, fungi fulfill an important ecological role and are being investigated as potential tools in bioremediation. For example, some species of fungi can be used to break down diesel oil and polycyclic aromatic hydrocarbons. Other species take up heavy metals such as cadmium and lead.
Fungal Diversity
The kingdom Fungi contains four major divisions that were established according to their mode of sexual reproduction. Polyphyletic, unrelated fungi that reproduce without a sexual cycle, are placed for convenience in a fifth division, and a sixth major fungal group that does not fit well with any of the previous five has recently been described. Not all mycologists agree with this scheme. Rapid advances in molecular biology and the sequencing of 18S rRNA (a component of ribosomes) continue to reveal new and different relationships between the various categories of fungi.
The traditional divisions of Fungi are the Chytridiomycota (chytrids), the Zygomycota(conjugated fungi), the Ascomycota (sac fungi), and the Basidiomycota (club fungi). An older classification scheme grouped fungi that strictly use asexual reproduction into Deuteromycota, a group that is no longer in use. The Glomeromycota belong to a newly described group (Figure \(5\)).
Pathogenic Fungi
Many fungi have negative impacts on other species, including humans and the organisms they depend on for food. Fungi may be parasites, pathogens, and, in a very few cases, predators.
Plant Parasites and Pathogens
The production of enough good-quality crops is essential to our existence. Plant diseases have ruined crops, bringing widespread famine. Most plant pathogens are fungi that cause tissue decay and eventual death of the host (Figure \(6\)). In addition to destroying plant tissue directly, some plant pathogens spoil crops by producing potent toxins. Fungi are also responsible for food spoilage and the rotting of stored crops. For example, the fungus Claviceps purpurea causes ergot, a disease of cereal crops (especially of rye). Although the fungus reduces the yield of cereals, the effects of the ergot’s alkaloid toxins on humans and animals are of much greater significance: In animals, the disease is referred to as ergotism. The most common signs and symptoms are convulsions, hallucination, gangrene, and loss of milk in cattle. The active ingredient of ergot is lysergic acid, which is a precursor of the drug LSD. Smuts, rusts, and powdery or downy mildew are other examples of common fungal pathogens that affect crops.
Aflatoxins are toxic and carcinogenic compounds released by fungi of the genus Aspergillus. Periodically, harvests of nuts and grains are tainted by aflatoxins, leading to massive recall of produce, sometimes ruining producers, and causing food shortages in developing countries.
Animal and Human Parasites and Pathogens
Fungi can affect animals, including humans, in several ways. Fungi attack animals directly by colonizing and destroying tissues. Humans and other animals can be poisoned by eating toxic mushrooms or foods contaminated by fungi. In addition, individuals who display hypersensitivity to molds and spores develop strong and dangerous allergic reactions. Fungal infections are generally very difficult to treat because, unlike bacteria, fungi are eukaryotes. Antibiotics only target prokaryotic cells, whereas compounds that kill fungi also adversely affect the eukaryotic animal host.
Many fungal infections (mycoses) are superficial and termed cutaneous (meaning “skin”) mycoses. They are usually visible on the skin of the animal. Fungi that cause the superficial mycoses of the epidermis, hair, and nails rarely spread to the underlying tissue (Figure \(7\)). These fungi are often misnamed “dermatophytes” from the Greek dermis skin and phyte plant, but they are not plants. Dermatophytes are also called “ringworms” because of the red ring that they cause on skin (although the ring is caused by fungi, not a worm). These fungi secrete extracellular enzymes that break down keratin (a protein found in hair, skin, and nails), causing a number of conditions such as athlete’s foot, jock itch, and other cutaneous fungal infections. These conditions are usually treated with over-the-counter topical creams and powders, and are easily cleared. More persistent, superficial mycoses may require prescription oral medications.
Systemic mycoses spread to internal organs, most commonly entering the body through the respiratory system. For example, coccidioidomycosis (valley fever) is commonly found in the southwestern United States, where the fungus resides in the dust. Once inhaled, the spores develop in the lungs and cause signs and symptoms similar to those of tuberculosis. Histoplasmosis (Figure \(7\)c) is caused by the dimorphic fungus Histoplasma capsulatum; it causes pulmonary infections and, in rare cases, swelling of the membranes of the brain and spinal cord. Treatment of many fungal diseases requires the use of antifungal medications that have serious side effects.
Opportunistic mycoses are fungal infections that are either common in all environments or part of the normal biota. They affect mainly individuals who have a compromised immune system. Patients in the late stages of AIDS suffer from opportunistic mycoses, such as Pneumocystis, which can be life threatening. The yeast Candida spp., which is a common member of the natural biota, can grow unchecked if the pH, the immune defenses, or the normal population of bacteria is altered, causing yeast infections of the vagina or mouth (oral thrush).
Fungi may even take on a predatory lifestyle. In soil environments that are poor in nitrogen, some fungi resort to predation of nematodes (small roundworms). Species of Arthrobotrys fungi have a number of mechanisms to trap nematodes. For example, they have constricting rings within their network of hyphae. The rings swell when the nematode touches it and closes around the body of the nematode, thus trapping it. The fungus extends specialized hyphae that can penetrate the body of the worm and slowly digest the hapless prey.
Beneficial Fungi
Fungi play a crucial role in the balance of ecosystems. They colonize most habitats on Earth, preferring dark, moist conditions. They can thrive in seemingly hostile environments, such as the tundra, thanks to a most successful symbiosis with photosynthetic organisms, like lichens. Fungi are not obvious in the way that large animals or tall trees are. Yet, like bacteria, they are major decomposers of nature. With their versatile metabolism, fungi break down organic matter that is insoluble and would not be recycled otherwise.
Importance to Ecosystems
Food webs would be incomplete without organisms that decompose organic matter and fungi are key participants in this process. Decomposition allows for cycling of nutrients such as carbon, nitrogen, and phosphorus back into the environment so they are available to living things, rather than being trapped in dead organisms. Fungi are particularly important because they have evolved enzymes to break down cellulose and lignin, components of plant cell walls that few other organisms are able to digest, releasing their carbon content.
Fungi are also involved in ecologically important coevolved symbioses, both mutually beneficial and pathogenic with organisms from the other kingdoms. Mycorrhiza, a term combining the Greek roots myco meaning fungus and rhizo meaning root, refers to the association between vascular plant roots and their symbiotic fungi. Somewhere between 80–90 percent of all plant species have mycorrhizal partners. In a mycorrhizal association, the fungal mycelia use their extensive network of hyphae and large surface area in contact with the soil to channel water and minerals from the soil into the plant. In exchange, the plant supplies the products of photosynthesis to fuel the metabolism of the fungus. Ectomycorrhizae (“outside” mycorrhiza) depend on fungi enveloping the roots in a sheath (called a mantle) and a net of hyphae that extends into the roots between cells. In a second type, the Glomeromycota fungi form arbuscular mycorrhiza. In these mycorrhiza, the fungi form arbuscles, a specialized highly branched hypha, which penetrate root cells and are the sites of the metabolic exchanges between the fungus and the host plant. Orchids rely on a third type of mycorrhiza. Orchids form small seeds without much storage to sustain germination and growth. Their seeds will not germinate without a mycorrhizal partner (usually Basidiomycota). After nutrients in the seed are depleted, fungal symbionts support the growth of the orchid by providing necessary carbohydrates and minerals. Some orchids continue to be mycorrhizal throughout their lifecycle.
Lichens blanket many rocks and tree bark, displaying a range of colors and textures. Lichens are important pioneer organisms that colonize rock surfaces in otherwise lifeless environments such as are created by glacial recession. The lichen is able to leach nutrients from the rocks and break them down in the first step to creating soil. Lichens are also present in mature habitats on rock surfaces or the trunks of trees. They are an important food source for caribou. Lichens are not a single organism, but rather a fungus (usually an Ascomycota or Basidiomycota species) living in close contact with a photosynthetic organism (an alga or cyanobacterium). The body of a lichen, referred to as a thallus, is formed of hyphae wrapped around the green partner. The photosynthetic organism provides carbon and energy in the form of carbohydrates and receives protection from the elements by the thallus of the fungal partner. Some cyanobacteria fix nitrogen from the atmosphere, contributing nitrogenous compounds to the association. In return, the fungus supplies minerals and protection from dryness and excessive light by encasing the algae in its mycelium. The fungus also attaches the symbiotic organism to the substrate.
Fungi have evolved mutualistic associations with numerous arthropods. The association between species of Basidiomycota and scale insects is one example. The fungal mycelium covers and protects the insect colonies. The scale insects foster a flow of nutrients from the parasitized plant to the fungus. In a second example, leaf-cutting ants of Central and South America literally farm fungi. They cut disks of leaves from plants and pile them up in gardens. Fungi are cultivated in these gardens, digesting the cellulose that the ants cannot break down. Once smaller sugar molecules are produced and consumed by the fungi, they in turn become a meal for the ants. The insects also patrol their garden, preying on competing fungi. Both ants and fungi benefit from the association. The fungus receives a steady supply of leaves and freedom from competition, while the ants feed on the fungi they cultivate.
Importance to Humans
Although we often think of fungi as organisms that cause diseases and rot food, fungi are important to human life on many levels. As we have seen, they influence the well-being of human populations on a large scale because they help nutrients cycle in ecosystems. They have other ecosystem roles as well. For example, as animal pathogens, fungi help to control the population of damaging pests. These fungi are very specific to the insects they attack and do not infect other animals or plants. The potential to use fungi as microbial insecticides is being investigated, with several species already on the market. For example, the fungus Beauveria bassiana is a pesticide that is currently being tested as a possible biological control for the recent spread of emerald ash borer. It has been released in Michigan, Illinois, Indiana, Ohio, West Virginia, and Maryland.
The mycorrhizal relationship between fungi and plant roots is essential for the productivity of farmland. Without the fungal partner in the root systems, 80–90% of trees and grasses would not survive. Mycorrhizal fungal inoculants are available as soil amendments from gardening supply stores and promoted by supporters of organic agriculture.
We also eat some types of fungi. Mushrooms figure prominently in the human diet. Morels, shiitake mushrooms, chanterelles, and truffles are considered delicacies (Figure \(8\)). The humble meadow mushroom, Agaricus campestris, appears in many dishes. Molds of the genus Penicillium ripen many cheeses. They originate in the natural environment such as the caves of Roquefort, France, where wheels of sheep milk cheese are stacked to capture the molds responsible for the blue veins and pungent taste of the cheese.
Fermentation—of grains to produce beer, and of fruits to produce wine—is an ancient art that humans in most cultures have practiced for millennia. Wild yeasts are acquired from the environment and used to ferment sugars into CO2 and ethyl alcohol under anaerobic conditions. It is now possible to purchase isolated strains of wild yeasts from different wine-making regions. Pasteur was instrumental in developing a reliable strain of brewer’s yeast, Saccharomyces cerevisiae, for the French brewing industry in the late 1850s. It was one of the first examples of biotechnology patenting. Yeast is also used to make breads that rise. The carbon dioxide they produce is responsible for the bubbles produced in the dough that become the air pockets of the baked bread.
Many secondary metabolites of fungi are of great commercial importance. Antibiotics are naturally produced by fungi to kill or inhibit the growth of bacteria, and limit competition in the natural environment. Valuable drugs isolated from fungi include the immunosuppressant drug cyclosporine (which reduces the risk of rejection after organ transplant), the precursors of steroid hormones, and ergot alkaloids used to stop bleeding. In addition, as easily cultured eukaryotic organisms, some fungi are important model research organisms including the red bread mold Neurospora crassa and the yeast, S. cerevisiae.
Section Summary
Fungi are eukaryotic organisms that appeared on land over 450 million years ago. They are heterotrophs and contain neither photosynthetic pigments such as chlorophylls nor organelles such as chloroplasts. Because they feed on decaying and dead matter, they are saprobes. Fungi are important decomposers and release essential elements into the environment. External enzymes digest nutrients that are absorbed by the body of the fungus called a thallus. A thick cell wall made of chitin surrounds the cell. Fungi can be unicellular as yeasts or develop a network of filaments called a mycelium, often described as mold. Most species multiply by asexual and sexual reproductive cycles, and display an alternation of generations.
The divisions of fungi are the Chytridiomycota, Zygomycota, Ascomycota, Basidiomycota, and Glomeromycota.
Fungi establish parasitic relationships with plants and animals. Fungal diseases can decimate crops and spoil food during storage. Compounds produced by fungi can be toxic to humans and other animals. Mycoses are infections caused by fungi. Superficial mycoses affect the skin, whereas systemic mycoses spread through the body. Fungal infections are difficult to cure.
Fungi have colonized all environments on Earth but are most often found in cool, dark, moist places with a supply of decaying material. Fungi are important decomposers because they are saprobes. Many successful mutualistic relationships involve a fungus and another organism. They establish complex mycorrhizal associations with the roots of plants. Lichens are a symbiotic relationship between a fungus and a photosynthetic organism, usually an alga or cyanobacterium.
Fungi are important to everyday human life. Fungi are important decomposers in most ecosystems. Mycorrhizal fungi are essential for the growth of most plants. Fungi, as food, play a role in human nutrition in the form of mushrooms and as agents of fermentation in the production of bread, cheeses, alcoholic beverages, and numerous other food preparations. Secondary metabolites of fungi are used in medicine as antibiotics and anticoagulants. Fungi are used in research as model organisms for the study of eukaryotic genetics and metabolism.
Glossary
Ascomycota
(sac fungi) a division of fungi that store spores in a sac called ascus
basidiomycota
(club fungi) a division of fungi that produce club shaped structures, basidia, which contain spores
Chytridiomycota
(chytrids) a primitive division of fungi that live in water and produce gametes with flagella
Glomeromycota
a group of fungi that form symbiotic relationships with the roots of trees
hypha
a fungal filament composed of one or more cells
lichen
the close association of a fungus with a photosynthetic alga or bacterium that benefits both partners
mold
a tangle of visible mycelia with a fuzzy appearance
mycelium
a mass of fungal hyphae
mycorrhiza
a mutualistic association between fungi and vascular plant roots
mycosis
a fungal infection
septum
the cell wall division between hyphae
thallus
a vegetative body of a fungus
yeast
a general term used to describe unicellular fungi
Zygomycota
(conjugated fungi) the division of fungi that form a zygote contained in a zygospore | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/31%3A_Fungi/31.01%3A_Classification_of_Fungi.txt |
Fungi are the major decomposers of nature; they break down organic matter which would otherwise not be recycled.
Learning Objectives
• Explain the roles played by fungi in decomposition and recycling
Key Points
• Aiding the survival of species from other kingdoms through the supply of nutrients, fungi play a major role as decomposers and recyclers in the wide variety of habitats in which they exist.
• Fungi provide a vital role in releasing scarce, yet biologically-essential elements, such as nitrogen and phosphorus, from decaying matter.
• Their mode of nutrition, which involves digestion before ingestion, allows fungi to degrade many large and insoluble molecules that would otherwise remain trapped in a habitat.
Key Terms
• decomposer: any organism that feeds off decomposing organic material, especially bacterium or fungi
• exoenzyme: any enzyme, generated by a cell, that functions outside of that cell
• saprobe: an organism that lives off of dead or decaying organic material
Fungi & Their Roles as Decomposers and Recyclers
Fungi play a crucial role in the balance of ecosystems. They colonize most habitats on earth, preferring dark, moist conditions. They can thrive in seemingly-hostile environments, such as the tundra. However, most members of the Kingdom Fungi grow on the forest floor where the dark and damp environment is rich in decaying debris from plants and animals. In these environments, fungi play a major role as decomposers and recyclers, making it possible for members of the other kingdoms to be supplied with nutrients and to live.
The food web would be incomplete without organisms that decompose organic matter. Some elements, such as nitrogen and phosphorus, are required in large quantities by biological systems; yet, they are not abundant in the environment. The action of fungi releases these elements from decaying matter, making them available to other living organisms. Trace elements present in low amounts in many habitats are essential for growth, but would remain tied up in rotting organic matter if fungi and bacteria did not return them to the environment via their metabolic activity.
The ability of fungi to degrade many large and insoluble molecules is due to their mode of nutrition. As seen earlier, digestion precedes ingestion. Fungi produce a variety of exoenzymes to digest nutrients. These enzymes are either released into the substrate or remain bound to the outside of the fungal cell wall. Large molecules are broken down into small molecules, which are transported into the cell by a system of protein carriers embedded in the cell membrane. Because the movement of small molecules and enzymes is dependent on the presence of water, active growth depends on a relatively-high percentage of moisture in the environment.
As saprobes, fungi help maintain a sustainable ecosystem for the animals and plants that share the same habitat. In addition to replenishing the environment with nutrients, fungi interact directly with other organisms in beneficial, but sometimes damaging, ways.
31.3B: Mutualistic Relationships with Fungi and Fungivores
Members of Kingdom Fungi form ecologically beneficial mutualistic relationships with cyanobateria, plants, and animals.
Learning Objectives
• Describe mutualistic relationships with fungi
Key Points
• Mutualistic relationships are those where both members of an association benefit; Fungi form these types of relationships with various other Kingdoms of life.
• Mycorrhiza, formed from an association between plant roots and primitive fungi, help increase a plant’s nutrient uptake; in return, the plant supplies the fungi with photosynthesis products for their metabolic use.
• In lichen, fungi live in close proximity with photosynthetic cyanobateria; the algae provide fungi with carbon and energy while the fungi supplies minerals and protection to the algae.
• Mutualistic relationships between fungi and animals involves numerous insects; Arthropods depend on fungi for protection, while fungi receive nutrients in return and ensure a way to disseminate the spores into new environments.
Key Terms
• mycorrhiza: a symbiotic association between a fungus and the roots of a vascular plant
• lichen: any of many symbiotic organisms, being associations of fungi and algae; often found as white or yellow patches on old walls, etc.
• thallus: vegetative body of a fungus
Mutualistic Relationships
Symbiosis is the ecological interaction between two organisms that live together. However, the definition does not describe the quality of the interaction. When both members of the association benefit, the symbiotic relationship is called mutualistic. Fungi form mutualistic associations with many types of organisms, including cyanobacteria, plants, and animals.
Fungi & Plant Mutualism
Mycorrhiza, which comes from the Greek words “myco” meaning fungus and “rhizo” meaning root, refers to the association between vascular plant roots and their symbiotic fungi. About 90 percent of all plant species have mycorrhizal partners. In a mycorrhizal association, the fungal mycelia use their extensive network of hyphae and large surface area in contact with the soil to channel water and minerals from the soil into the plant, thereby increasing a plant’s nutrient uptake. In exchange, the plant supplies the products of photosynthesis to fuel the metabolism of the fungus.
Mycorrhizae display many characteristics of primitive fungi: they produce simple spores, show little diversification, do not have a sexual reproductive cycle, and cannot live outside of a mycorrhizal association. There are a number of types of mycorrhizae. Ectomycorrhizae (“outside” mycorrhiza) depend on fungi enveloping the roots in a sheath (called a mantle) and a Hartig net of hyphae that extends into the roots between cells. The fungal partner can belong to the Ascomycota, Basidiomycota, or Zygomycota. In a second type, the Glomeromycete fungi form vesicular–arbuscular interactions with arbuscular mycorrhiza (sometimes called endomycorrhizae). In these mycorrhiza, the fungi form arbuscules that penetrate root cells and are the site of the metabolic exchanges between the fungus and the host plant. The arbuscules (from the Latin for “little trees”) have a shrub-like appearance. Orchids rely on a third type of mycorrhiza. Orchids are epiphytes that form small seeds without much storage to sustain germination and growth. Their seeds will not germinate without a mycorrhizal partner (usually a Basidiomycete). After nutrients in the seed are depleted, fungal symbionts support the growth of the orchid by providing necessary carbohydrates and minerals. Some orchids continue to be mycorrhizal throughout their lifecycle.
Lichens
Lichens display a range of colors and textures. They can survive in the most unusual and hostile habitats. They cover rocks, gravestones, tree bark, and the ground in the tundra where plant roots cannot penetrate. Lichens can survive extended periods of drought: they become completely desiccated and then rapidly become active once water is available again. Lichens fulfill many ecological roles, including acting as indicator species, which allow scientists to track the health of a habitat because of their sensitivity to air pollution.
Lichens are not a single organism, but, rather, an example of a mutualism in which a fungus (usually a member of the Ascomycota or Basidiomycota phyla) lives in close contact with a photosynthetic organism (a eukaryotic alga or a prokaryotic cyanobacterium). Generally, neither the fungus nor the photosynthetic organism can survive alone outside of the symbiotic relationship. The body of a lichen, referred to as a thallus, is formed of hyphae wrapped around the photosynthetic partner. The photosynthetic organism provides carbon and energy in the form of carbohydrates. Some cyanobacteria fix nitrogen from the atmosphere, contributing nitrogenous compounds to the association. In return, the fungus supplies minerals and protection from dryness and excessive light by encasing the algae in its mycelium. The fungus also attaches the symbiotic organism to the substrate.
The thallus of lichens grows very slowly, expanding its diameter a few millimeters per year. Both the fungus and the alga participate in the formation of dispersal units for reproduction. Lichens produce soredia, clusters of algal cells surrounded by mycelia. Soredia are dispersed by wind and water and form new lichens.
Fungi & Animal Mutualism
Fungi have evolved mutualisms with numerous insects. Arthropods (jointed, legged invertebrates, such as insects) depend on the fungus for protection from predators and pathogens, while the fungus obtains nutrients and a way to disseminate spores into new environments. The association between species of Basidiomycota and scale insects is one example. The fungal mycelium covers and protects the insect colonies. The scale insects foster a flow of nutrients from the parasitized plant to the fungus. In a second example, leaf-cutting ants of Central and South America literally farm fungi. They cut disks of leaves from plants and pile them up in gardens. Fungi are cultivated in these disk gardens, digesting the cellulose in the leaves that the ants cannot break down. Once smaller sugar molecules are produced and consumed by the fungi, the fungi in turn become a meal for the ants. The insects also patrol their garden, preying on competing fungi. Both ants and fungi benefit from the association. The fungus receives a steady supply of leaves and freedom from competition, while the ants feed on the fungi they cultivate. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/31%3A_Fungi/31.03%3A_Fungal_Ecology/31.3A%3A_Fungi_Habitat_Decomposition_and_Recycling.txt |
Would you eat these mushrooms?
I would not recommend it. But certain red mushrooms, Ganoderma Lucidum, have been found to be good for you. Red Mushrooms comprise a family of more than 200 mushroom species, which are good for our health. Of these, 6 species have a particularly high therapeutic effect.
Fungi and Human Disease
Fungi cause human illness in three different ways: poisonings, parasitic infections, and allergic reactions. Science on the SPOT: Fungus Fair explores some of these dangerous but also tasty and weirdly wonderful fungi.
Fungal Poisoning
Many fungi protect themselves from parasites and predators by producing toxic chemicals. If people eat toxic fungi, they may experience digestive problems, hallucinations, organ failure, and even death. Most cases of mushroom poisoning are due to mistaken identity. That’s because many toxic mushrooms look very similar to safe, edible mushrooms. An example is shown in Figure below.
Poisonous or Edible? The destroying angel mushroom on the left causes liver and kidney failure. The puffball mushroom on the right is tasty and harmless. Do you think you could tell these two species of mushrooms apart?
Fungal Parasites
Some fungi cause disease when they become human parasites. Two examples are fungi in the genera Candida and Trichophyton.
• Candida are yeast that cause candidiasis, commonly called a “yeast infection.” The yeast can infect the mouth or the vagina. If yeast enter the blood, they cause a potentially life threatening illness. However, this is rare, except in people with a depressed immune system.
• Trichophyton are fungi that cause ringworm. This is a skin infection characterized by a ring-shaped rash. The rash may occur on the arms, legs, head, neck, or trunk. The same fungi cause athlete’s foot when they infect the skin between the toes. Athlete’s foot is the second most common skin disease in the U.S.
Figure below shows signs of these two infections.
Ringworm produces a ring-shaped rash, but it isn’t caused by a worm. It’s caused by the same fungus that causes athlete’s foot.
Fungal Allergies
Mold allergies are very common. They are caused by airborne mold spores. When the spores enter the respiratory tract, the immune system responds to them as though they were harmful microbes. Symptoms may include sneezing, coughing, and difficulty breathing. The symptoms are likely to be more severe in people with asthma or other respiratory diseases. Long-term exposure to mold spores may also weaken the immune system.
Molds grow indoors as well as out. Indoors, they grow in showers, basements, and other damp places. Homes damaged in floods and hurricanes may have mold growing just about everywhere (see Figure below). Indoor mold may cause more health problems than outdoor mold because of the closed, confined space. Most people also spend more time indoors than out.
The mold growing on the walls and ceiling of this storm-damaged home may be harmful to human health.
Summary
• Fungi cause three different types of human illness: poisonings, parasitic infections, and allergies.
• Many poisonous mushrooms are eaten by mistake because they look like edible mushrooms.
• Parasitic yeasts cause candidiasis, ringworm, and athlete’s foot.
• Mold allergies are very common.
Review
1. Explain why you should never eat mushrooms you find in the woods unless you know for certain which type of mushrooms they are.
2. Compare and contrast ringworm and athlete’s foot.
3. How does mold cause allergies?
4. State why indoor mold may cause more health problems than outdoor mold. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/31%3A_Fungi/31.04%3A_Fungal_Parasites_and_Pathogens.txt |
The basidiomycota are mushroom-producing fungi with developing, club-shaped fruiting bodies called basidia on the gills under its cap.
Learning Objectives
• Describe the ecology and reproduction of the Basidiomycota
Key Points
• The majority of edible fungi belong to the Phylum Basidiomycota.
• The basidiomycota includes shelf fungus, toadstools, and smuts and rusts.
• Unlike most fungi, basidiomycota reproduce sexually as opposed to asexually.
• Two different mating strains are required for the fusion of genetic material in the basidium which is followed by meiosis producing haploid basidiospores.
• Mycelia of different mating strains combine to produce a secondary mycelium that contains haploid basidiospores in what is called the dikaryotic stage, where the fungi remains until a basidiocarp (mushroom) is generated with the developing basidia on the gills under its cap.
Key Terms
• basidiocarp: a fruiting body that protrudes from the ground, known as a mushroom, which has a developing basidia on the gills under its cap
• basidiomycete: a fungus of the phylum Basidiomycota, which produces sexual spores on a basidium
• Basidiomycota: a taxonomic division within the kingdom Fungi: 30,000 species of fungi that produce spores from a basidium
• basidium: a small structure, shaped like a club, found in the Basidiomycota phylum of fungi, that bears four spores at the tips of small projections
• basidiospore: a sexually-reproductive spore produced by fungi of the phylum Basidiomycota
Basidiomycota: The Club Fungi
The fungi in the Phylum Basidiomycota are easily recognizable under a light microscope by their club-shaped fruiting bodies called basidia (singular, basidium), which are the swollen terminal cell of a hypha. The basidia, which are the reproductive organs of these fungi, are often contained within the familiar mushroom, commonly seen in fields after rain, on the supermarket shelves, and growing on your lawn. These mushroom-producing basidiomyces are sometimes referred to as “gill fungi” because of the presence of gill-like structures on the underside of the cap. The “gills” are actually compacted hyphae on which the basidia are borne. This group also includes shelf fungus, which cling to the bark of trees like small shelves. In addition, the basidiomycota includes smuts and rusts, which are important plant pathogens, and toadstools. Most edible fungi belong to the Phylum Basidiomycota; however, some basidiomycetes produce deadly toxins. For example, Cryptococcus neoformans causes severe respiratory illness.
The lifecycle of basidiomycetes includes alternation of generations. Spores are generally produced through sexual reproduction, rather than asexual reproduction. The club-shaped basidium carries spores called basidiospores. In the basidium, nuclei of two different mating strains fuse (karyogamy), giving rise to a diploid zygote that then undergoes meiosis. The haploid nuclei migrate into basidiospores, which germinate and generate monokaryotic hyphae. The mycelium that results is called a primary mycelium. Mycelia of different mating strains can combine and produce a secondary mycelium that contains haploid nuclei of two different mating strains. This is the dikaryotic stage of the basidiomyces lifecyle and it is the dominant stage. Eventually, the secondary mycelium generates a basidiocarp, which is a fruiting body that protrudes from the ground; this is what we think of as a mushroom. The basidiocarp bears the developing basidia on the gills under its cap. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/31%3A_Fungi/31.05%3A_Basidomycota-_The_Club_%28Basidium%29_Fungi.txt |
Learning Objectives
• Describe the ecology and the reproduction of Ascomycetes
The majority of known fungi belong to the Phylum Ascomycota, which is characterized by the formation of an ascus (plural, asci), a sac-like structure that contains haploid ascospores. Many ascomycetes are of commercial importance. Some play a beneficial role, such as the yeasts used in baking, brewing, and wine fermentation, plus truffles and morels, which are held as gourmet delicacies. Aspergillus oryzae is used in the fermentation of rice to produce sake. Other ascomycetes parasitize plants and animals, including humans. For example, fungal pneumonia poses a significant threat to AIDS patients who have a compromised immune system. Ascomycetes not only infest and destroy crops directly, they also produce poisonous secondary metabolites that make crops unfit for consumption. Filamentous ascomycetes produce hyphae divided by perforated septa, allowing streaming of cytoplasm from one cell to the other. Conidia and asci, which are used respectively for asexual and sexual reproductions, are usually separated from the vegetative hyphae by blocked (non-perforated) septa.
Asexual reproduction is frequent and involves the production of conidiophores that release haploid conidiospores. Sexual reproduction starts with the development of special hyphae from either one of two types of mating strains. The “male” strain produces an antheridium (plural: antheridia) and the “female” strain develops an ascogonium (plural: ascogonia). At fertilization, the antheridium and the ascogonium combine in plasmogamy without nuclear fusion. Special ascogenous hyphae arise, in which pairs of nuclei migrate: one from the “male” strain and one from the “female” strain. In each ascus, two or more haploid ascospores fuse their nuclei in karyogamy. During sexual reproduction, thousands of asci fill a fruiting body called the ascocarp. The diploid nucleus gives rise to haploid nuclei by meiosis. The ascospores are then released, germinate, and form hyphae that are disseminated in the environment and start new mycelia.
Key Points
• Ascomycota fungi are the yeasts used in baking, brewing, and wine fermentation, plus delicacies such as truffles and morels.
• Ascomycetes are filamentous and produce hyphae divided by perforated septa.
• Ascomycetes frequently reproduce asexually which leads to the production of conidiophores that release haploid conidiospores.
• Two types of mating strains, a “male” strain which produces an antheridium and a “female” strain which develops an ascogonium, are required for sexual reproduction.
• The antheridium and the ascogonium combine in plasmogamy at the time of fertilization, followed by nuclei fusion in the asci.
• In the ascocarp, a fruiting body, thousands of asci undergo meiosis to generate haploid ascospores ready to be released to the world.
Key Terms
• plasmogamy: stage of sexual reproduction joining the cytoplasm of two parent mycelia without the fusion of nuclei
• Ascomycota: a taxonomic division within the kingdom Fungi; those fungi that produce spores in a microscopic sporangium called an ascus
• ascus: a sac-shaped cell present in ascomycete fungi; it is a reproductive cell in which meiosis and an additional cell division produce eight spores
• ascospore: a sexually-produced spore from the ascus of an Ascomycetes fungus
• conidia: asexual, non-motile spores of a fungus, named after the Greek word for dust, conia; also known as conidiospores and mitospores
• antheridia: a haploid structure or organ producing and containing male gametes (called antherozoids or sperm) present in lower plants like mosses and ferns, primitive vascular psilotophytes, and fungi
• ascogonium: a haploid structure or organ producing and containing female gametes in certain Ascomycota fungi
• ascocarp: the sporocarp of an ascomycete, typically bowl-shaped
• ascomycete: any fungus of the phylum Ascomycota, characterized by the production of a sac, or ascus, which contains non-motile spores
31.07: Glomeromycota- Asexual Plant Symbionts
Learning Objectives
• Describe the ecology and reproduction of Glomeromycetes
In the kingdom Fungi, the Glomeromycota is a newly-established phylum comprised of about 230 species that live in close association with the roots of trees and plants. Fossil records indicate that trees and their root symbionts share a long evolutionary history. It appears that most members of this family form arbuscular mycorrhizae: the hyphae interact with the root cells forming a mutually-beneficial association where the plants supply the carbon source and energy in the form of carbohydrates to the fungus while the fungus supplies essential minerals from the soil to the plant. This association is termed biotrophic. The Glomeromycota species that have arbuscular mycorrhizal are terrestrial and widely distributed in soils worldwide where they form symbioses with the roots of the majority of plant species. They can also be found in wetlands, including salt-marshes, and are associated with epiphytic plants.
The glomeromycetes do not reproduce sexually and cannot survive without the presence of plant roots. They have coenocytic hyphae and reproduce asexually, producing glomerospores. The biochemical and genetic characterization of the Glomeromycota has been hindered by their biotrophic nature, which impedes laboratory culturing. This obstacle was eventually surpassed with the use of root cultures. With the advent of molecular techniques, such as gene sequencing, the phylogenetic classification of Glomeromycota has become clearer. The first mycorrhizal gene to be sequenced was the small-subunit ribosomal RNA (SSU rRNA). This gene is highly conserved and commonly used in phylogenetic studies so it was isolated from spores of each taxonomic group. Using a molecular clock approach based on the substitution rates of SSU sequences, scientists were able to estimate the time of divergence of the fungi. This analysis shows that all glomeromycetes probably descended from a common ancestor 462 and 353 million years ago, making them a monophyletic lineage. A long-held theory is that Glomeromycota were instrumental in the colonization of land by plants.
Key Points
• Most glomeromycetes form arbuscular mycorrhizae, a type of symbiotic relationship between a fungus and plant roots; the plants supply a source of energy to the fungus while the fungus supplies essential minerals to the plant.
• Glomeromycota that have arbuscular mycorrhizal are mostly terrestrial, but can also be found in wetlands.
• The glomeromycetes reproduce asexually by producing glomerospores and cannot survive without the presence of plant roots.
• DNA analysis shows that all glomeromycetes probably descended from a common ancestor 462 and 353 million years ago.
• The classification of fungi as Glomeromycota has been redefined with adoption of molecular techniques.
Key Terms
• biotrophic: describing a parasite that needs its host to stay alive
• arbuscular mycorrhizae: a type of symbiotic relationship between a fungus and the roots of a plant where the plants supply a source of energy to the fungus while the fungus supplies essential minerals to the plant
• glomeromycete: an organism of the phylum Glomeromycota
LICENSES AND ATTRIBUTIONS
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Learning Objectives
• Describe the ecology and reproduction of Zygomycetes
The zygomycetes are a relatively small group in the fungi kingdom and belong to the Phylum Zygomycota. They include the familiar bread mold, Rhizopus stolonifer, which rapidly propagates on the surfaces of breads, fruits, and vegetables. They are mostly terrestrial in habitat, living in soil or on plants and animals. Most species are saprobes meaning they live off decaying organic material. Some are parasites of plants, insects, and small animals, while others form symbiotic relationships with plants. Zygomycetes play a considerable commercial role. The metabolic products of other species of Rhizopus are intermediates in the synthesis of semi-synthetic steroid hormones.
Zygomycetes have a thallus of coenocytic hyphae in which the nuclei are haploid when the organism is in the vegetative stage. The fungi usually reproduce asexually by producing sporangiospores. The black tips of bread mold, Rhizopus stolonifer, are the swollen sporangia packed with black spores. When spores land on a suitable substrate, they germinate and produce a new mycelium.
Sexual reproduction starts when conditions become unfavorable. Two opposing mating strains (type + and type –) must be in close proximity for gametangia (singular: gametangium) from the hyphae to be produced and fuse, leading to karyogamy. The developing diploid zygospores have thick coats that protect them from desiccation and other hazards. They may remain dormant until environmental conditions become favorable. When the zygospore germinates, it undergoes meiosis and produces haploid spores, which will, in turn, grow into a new organism. This form of sexual reproduction in fungi is called conjugation (although it differs markedly from conjugation in bacteria and protists), giving rise to the name “conjugated fungi”.
Key Points
• Most zygomycota are saprobes, while a few species are parasites.
• Zygomycota usually reproduce asexually by producing sporangiospores.
• Zygomycota reproduce sexually when environmental conditions become unfavorable.
• To reproduce sexually, two opposing mating strains must fuse or conjugate, thereby, sharing genetic content and creating zygospores.
• The resulting diploid zygospores remain dormant and protected by thick coats until environmental conditions have improved.
• When conditions become favorable, zygospores undergo meiosis to produce haploid spores, which will eventually grow into a new organism.
Key Terms
• zygomycete: an organism of the phylum Zygomycota
• karyogamy: the fusion of two nuclei within a cell
• zygospore: a spore formed by the union of several zoospores
• conjugation: the temporary fusion of organisms, especially as part of sexual reproduction
31.09: Chytridmycota and Relatives- Fungi with Zoospores
Chytrids are the most primitive group of fungi and the only group that possess gametes with flagella.
Learning Objectives
• Describe the ecology and reproduction of chytrids
Key Points
• The first recognizable chytrids appeared more than 500 million years ago during the late pre-Cambrian period.
• Like protists, chytrids usually live in aquatic environments, but some species live on land.
• Some chytrids are saprobes while others are parasites that may be harmful to amphibians and other animals.
• Chytrids reproduce both sexually and asexually, which leads to the production of zoospores.
• Chytrids have chitin in their cell walls; one unique group also has cellulose along with chitin.
• Chytrids are mostly unicellular, but multicellular organisms do exist.
Key Terms
• chytridiomycete: an organism of the phylum Chytridiomycota
• zoospore: an asexual spore of some algae and fungi
• flagellum: a flagellum is a lash-like appendage that protrudes from the cell body of certain prokaryotic and eukaryotic cells
• coenocytic: a multinucleate cell that can result from multiple nuclear divisions without their accompanying cytokinesis
Chytridiomycota: The Chytrids
The kingdom Fungi contains five major phyla, which were established according to their mode of sexual reproduction or use of molecular data. The Phylum Chytridiomycota (chytrids) is one of the five true phyla of fungi. There is only one class in the Phylum Chytridiomycota, the Chytridiomycetes. The chytrids are the simplest and most primitive Eumycota, or true fungi. The evolutionary record shows that the first, recognizable chytrids appeared during the late pre-Cambrian period, more than 500 million years ago. Like all fungi, chytrids have chitin in their cell walls, but one group of chytrids has both cellulose and chitin in the cell wall. Most chytrids are unicellular; a few form multicellular organisms and hyphae, which have no septa between cells (coenocytic). They reproduce both sexually and asexually; the asexual spores are called diploid zoospores. Their gametes are the only fungal cells known to have a flagellum.
The ecological habitat and cell structure of chytrids have much in common with protists. Chytrids usually live in aquatic environments, although some species live on land. Some species thrive as parasites on plants, insects, or amphibians, while others are saprobes. Some chytrids cause diseases in many species of amphibians, resulting in species decline and extinction. An example of a harmful parasitic chytrid is Batrachochytrium dendrobatidis, which is known to cause skin disease. Another chytrid species, Allomyces, is well characterized as an experimental organism. Its reproductive cycle includes both asexual and sexual phases. Allomyces produces diploid or haploid flagellated zoospores in a sporangium. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/31%3A_Fungi/31.08%3A_Zygomycota-_Zygote-Producing_Fungi.txt |
Skills to Develop
• List the features that distinguish the kingdom Animalia from other kingdoms
• Explain the processes of animal reproduction and embryonic development
• Describe the roles that Hox genes play in development
Even though members of the animal kingdom are incredibly diverse, most animals share certain features that distinguish them from organisms in other kingdoms. All animals are eukaryotic, multicellular organisms, and almost all animals have a complex tissue structure with differentiated and specialized tissues. Most animals are motile, at least during certain life stages. All animals require a source of food and are therefore heterotrophic, ingesting other living or dead organisms; this feature distinguishes them from autotrophic organisms, such as most plants, which synthesize their own nutrients through photosynthesis. As heterotrophs, animals may be carnivores, herbivores, omnivores, or parasites (Figure \(1\)). Most animals reproduce sexually, and the offspring pass through a series of developmental stages that establish a determined and fixed body plan. The body plan refers to the morphology of an animal, determined by developmental cues.
Complex Tissue Structure
As multicellular organisms, animals differ from plants and fungi because their cells don’t have cell walls, their cells may be embedded in an extracellular matrix (such as bone, skin, or connective tissue), and their cells have unique structures for intercellular communication (such as gap junctions). In addition, animals possess unique tissues, absent in fungi and plants, which allow coordination (nerve tissue) of motility (muscle tissue). Animals are also characterized by specialized connective tissues that provide structural support for cells and organs. This connective tissue constitutes the extracellular surroundings of cells and is made up of organic and inorganic materials. In vertebrates, bone tissue is a type of connective tissue that supports the entire body structure. The complex bodies and activities of vertebrates demand such supportive tissues. Epithelial tissues cover, line, protect, and secrete. Epithelial tissues include the epidermis of the integument, the lining of the digestive tract and trachea, and make up the ducts of the liver and glands of advanced animals.
The animal kingdom is divided into Parazoa (sponges) and Eumetazoa (all other animals). As very simple animals, the organisms in group Parazoa (“beside animal”) do not contain true specialized tissues; although they do possess specialized cells that perform different functions, those cells are not organized into tissues. These organisms are considered animals since they lack the ability to make their own food. Animals with true tissues are in the group Eumetazoa (“true animals”). When we think of animals, we usually think of Eumetazoans, since most animals fall into this category.
The different types of tissues in true animals are responsible for carrying out specific functions for the organism. This differentiation and specialization of tissues is part of what allows for such incredible animal diversity. For example, the evolution of nerve tissues and muscle tissues has resulted in animals’ unique ability to rapidly sense and respond to changes in their environment. This allows animals to survive in environments where they must compete with other species to meet their nutritional demands.
Link to Learning
Watch a presentation by biologist E.O. Wilson on the importance of diversity.
Animal Reproduction and Development
Most animals are diploid organisms, meaning that their body (somatic) cells are diploid and haploid reproductive (gamete) cells are produced through meiosis. Some exceptions exist: For example, in bees, wasps, and ants, the male is haploid because it develops from unfertilized eggs. Most animals undergo sexual reproduction: This fact distinguishes animals from fungi, protists, and bacteria, where asexual reproduction is common or exclusive. However, a few groups, such as cnidarians, flatworm, and roundworms, undergo asexual reproduction, although nearly all of those animals also have a sexual phase to their life cycle.
Processes of Animal Reproduction and Embryonic Development
During sexual reproduction, the haploid gametes of the male and female individuals of a species combine in a process called fertilization. Typically, the small, motile male sperm fertilizes the much larger, sessile female egg. This process produces a diploid fertilized egg called a zygote.
Some animal species—including sea stars and sea anemones, as well as some insects, reptiles, and fish—are capable of asexual reproduction. The most common forms of asexual reproduction for stationary aquatic animals include budding and fragmentation, where part of a parent individual can separate and grow into a new individual. In contrast, a form of asexual reproduction found in certain insects and vertebrates is called parthenogenesis (or “virgin beginning”), where unfertilized eggs can develop into new male offspring. This type of parthenogenesis is called haplodiploidy. These types of asexual reproduction produce genetically identical offspring, which is disadvantageous from the perspective of evolutionary adaptability because of the potential buildup of deleterious mutations. However, for animals that are limited in their capacity to attract mates, asexual reproduction can ensure genetic propagation.
After fertilization, a series of developmental stages occur during which primary germ layers are established and reorganize to form an embryo. During this process, animal tissues begin to specialize and organize into organs and organ systems, determining their future morphology and physiology. Some animals, such as grasshoppers, undergo incomplete metamorphosis, in which the young resemble the adult. Other animals, such as some insects, undergo complete metamorphosis where individuals enter one or more larval stages that may in differ in structure and function from the adult (Figure \(2\)). For the latter, the young and the adult may have different diets, limiting competition for food between them. Regardless of whether a species undergoes complete or incomplete metamorphosis, the series of developmental stages of the embryo remains largely the same for most members of the animal kingdom.
The process of animal development begins with the cleavage, or series of mitotic cell divisions, of the zygote (Figure \(3\)). Three cell divisions transform the single-celled zygote into an eight-celled structure. After further cell division and rearrangement of existing cells, a 6–32-celled hollow structure called a blastula is formed. Next, the blastula undergoes further cell division and cellular rearrangement during a process called gastrulation. This leads to the formation of the next developmental stage, the gastrula, in which the future digestive cavity is formed. Different cell layers (called germ layers) are formed during gastrulation. These germ layers are programmed to develop into certain tissue types, organs, and organ systems during a process called organogenesis.
Link to Learning
Watch the following video to see how human embryonic development (after the blastula and gastrula stages of development) reflects evolution.
The Role of Homeobox (Hox) Genes in Animal Development
Since the early 19th century, scientists have observed that many animals, from the very simple to the complex, shared similar embryonic morphology and development. Surprisingly, a human embryo and a frog embryo, at a certain stage of embryonic development, look remarkably alike. For a long time, scientists did not understand why so many animal species looked similar during embryonic development but were very different as adults. They wondered what dictated the developmental direction that a fly, mouse, frog, or human embryo would take. Near the end of the 20th century, a particular class of genes was discovered that had this very job. These genes that determine animal structure are called “homeotic genes,” and they contain DNA sequences called homeoboxes. The animal genes containing homeobox sequences are specifically referred to as Hox genes. This family of genes is responsible for determining the general body plan, such as the number of body segments of an animal, the number and placement of appendages, and animal head-tail directionality. The first Hox genes to be sequenced were those from the fruit fly (Drosophila melanogaster). A single Hox mutation in the fruit fly can result in an extra pair of wings or even appendages growing from the “wrong” body part.
While there are a great many genes that play roles in the morphological development of an animal, what makes Hox genes so powerful is that they serve as master control genes that can turn on or off large numbers of other genes. Hox genes do this by coding transcription factors that control the expression of numerous other genes. Hox genes are homologous in the animal kingdom, that is, the genetic sequences of Hox genes and their positions on chromosomes are remarkably similar across most animals because of their presence in a common ancestor, from worms to flies, mice, and humans (Figure \(4\)). One of the contributions to increased animal body complexity is that Hox genes have undergone at least two duplication events during animal evolution, with the additional genes allowing for more complex body types to evolve.
Art Connection
If a Hox 13 gene in a mouse was replaced with a Hox 1 gene, how might this alter animal development?
Summary
Animals constitute an incredibly diverse kingdom of organisms. Although animals range in complexity from simple sea sponges to human beings, most members of the animal kingdom share certain features. Animals are eukaryotic, multicellular, heterotrophic organisms that ingest their food and usually develop into motile creatures with a fixed body plan. A major characteristic unique to the animal kingdom is the presence of differentiated tissues, such as nerve, muscle, and connective tissues, which are specialized to perform specific functions. Most animals undergo sexual reproduction, leading to a series of developmental embryonic stages that are relatively similar across the animal kingdom. A class of transcriptional control genes called Hox genes directs the organization of the major animal body plans, and these genes are strongly homologous across the animal kingdom.
Art Connections
Figure \(4\): If a Hox 13 gene in a mouse was replaced with a Hox 1 gene, how might this alter animal development?
Answer
The animal might develop two heads and no tail.
Glossary
blastula
16–32 cell stage of development of an animal embryo
body plan
morphology or constant shape of an organism
cleavage
cell division of a fertilized egg (zygote) to form a multicellular embryo
gastrula
stage of animal development characterized by the formation of the digestive cavity
germ layer
collection of cells formed during embryogenesis that will give rise to future body tissues, more pronounced in vertebrate embryogenesis
Hox gene
(also, homeobox gene) master control gene that can turn on or off large numbers of other genes during embryogenesis
organogenesis
formation of organs in animal embryogenesis | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/32%3A_Animal_Diversity_and_the_Evolution_of_Body_Plans/32.01%3A_Some_General_Features_of_Animals.txt |
Skills to Develop
• Explain the differences in animal body plans that support basic animal classification
• Compare and contrast the embryonic development of protostomes and deuterostomes
Scientists have developed a classification scheme that categorizes all members of the animal kingdom, although there are exceptions to most “rules” governing animal classification (Figure \(1\)). Animals are primarily classified according to morphological and developmental characteristics, such as a body plan. One of the most prominent features of the body plan of true animals is that they are morphologically symmetrical. This means that their distribution of body parts is balanced along an axis. Additional characteristics include the number of tissue layers formed during development, the presence or absence of an internal body cavity, and other features of embryological development, such as the origin of the mouth and anus.
Art Connection
Which of the following statements is false?
1. Eumetazoans have specialized tissues and parazoans don’t.
2. Lophotrochozoa and Ecdysozoa are both Bilataria.
3. Acoela and Cnidaria both possess radial symmetry.
4. Arthropods are more closely related to nematodes than they are to annelids.
Animal Characterization Based on Body Symmetry
At a very basic level of classification, true animals can be largely divided into three groups based on the type of symmetry of their body plan: radially symmetrical, bilaterally symmetrical, and asymmetrical. Asymmetry is a unique feature of Parazoa (Figure \(2\)). Only a few animal groups display radial symmetry. All types of symmetry are well suited to meet the unique demands of a particular animal’s lifestyle.
Radial symmetry is the arrangement of body parts around a central axis, as is seen in a drinking glass or pie. It results in animals having top and bottom surfaces but no left and right sides, or front or back. The two halves of a radially symmetrical animal may be described as the side with a mouth or “oral side,” and the side without a mouth (the “aboral side”). This form of symmetry marks the body plans of animals in the phyla Ctenophora and Cnidaria, including jellyfish and adult sea anemones (Figure 27.2.2). Radial symmetry equips these sea creatures (which may be sedentary or only capable of slow movement or floating) to experience the environment equally from all directions.
Bilateral symmetry involves the division of the animal through a sagittal plane, resulting in two mirror image, right and left halves, such as those of a butterfly (Figure \(2\)), crab, or human body. Animals with bilateral symmetry have a “head” and “tail” (anterior vs. posterior), front and back (dorsal vs. ventral), and right and left sides (Figure \(3\)). All true animals except those with radial symmetry are bilaterally symmetrical. The evolution of bilateral symmetry that allowed for the formation of anterior and posterior (head and tail) ends promoted a phenomenon called cephalization, which refers to the collection of an organized nervous system at the animal’s anterior end. In contrast to radial symmetry, which is best suited for stationary or limited-motion lifestyles, bilateral symmetry allows for streamlined and directional motion. In evolutionary terms, this simple form of symmetry promoted active mobility and increased sophistication of resource-seeking and predator-prey relationships.
Animals in the phylum Echinodermata (such as sea stars, sand dollars, and sea urchins) display radial symmetry as adults, but their larval stages exhibit bilateral symmetry. This is termed secondary radial symmetry. They are believed to have evolved from bilaterally symmetrical animals; thus, they are classified as bilaterally symmetrical.
Link to Learning
Watch this video to see a quick sketch of the different types of body symmetry.
Animal Characterization Based on Features of Embryological Development
Most animal species undergo a separation of tissues into germ layers during embryonic development. Recall that these germ layers are formed during gastrulation, and that they are predetermined to develop into the animal’s specialized tissues and organs. Animals develop either two or three embryonic germs layers (Figure \(4\)). The animals that display radial symmetry develop two germ layers, an inner layer (endoderm) and an outer layer (ectoderm). These animals are called diploblasts. Diploblasts have a non-living layer between the endoderm and ectoderm. More complex animals (those with bilateral symmetry) develop three tissue layers: an inner layer (endoderm), an outer layer (ectoderm), and a middle layer (mesoderm). Animals with three tissue layers are called triploblasts.
Art Connection
Which of the following statements about diploblasts and triploblasts is false?
1. Animals that display radial symmetry are diploblasts.
2. Animals that display bilateral symmetry are triploblasts.
3. The endoderm gives rise to the lining of the digestive tract and the respiratory tract.
4. The mesoderm gives rise to the central nervous system.
Each of the three germ layers is programmed to give rise to particular body tissues and organs. The endoderm gives rise to the lining of the digestive tract (including the stomach, intestines, liver, and pancreas), as well as to the lining of the trachea, bronchi, and lungs of the respiratory tract, along with a few other structures. The ectoderm develops into the outer epithelial covering of the body surface, the central nervous system, and a few other structures. The mesoderm is the third germ layer; it forms between the endoderm and ectoderm in triploblasts. This germ layer gives rise to all muscle tissues (including the cardiac tissues and muscles of the intestines), connective tissues such as the skeleton and blood cells, and most other visceral organs such as the kidneys and the spleen.
Presence or Absence of a Coelom
Further subdivision of animals with three germ layers (triploblasts) results in the separation of animals that may develop an internal body cavity derived from mesoderm, called a coelom, and those that do not. This epithelial cell-lined coelomic cavity represents a space, usually filled with fluid, which lies between the visceral organs and the body wall. It houses many organs such as the digestive system, kidneys, reproductive organs, and heart, and contains the circulatory system. In some animals, such as mammals, the part of the coelom called the pleural cavity provides space for the lungs to expand during breathing. The evolution of the coelom is associated with many functional advantages. Primarily, the coelom provides cushioning and shock absorption for the major organ systems. Organs housed within the coelom can grow and move freely, which promotes optimal organ development and placement. The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility, promoting improved animal motility.
Triploblasts that do not develop a coelom are called acoelomates, and their mesoderm region is completely filled with tissue, although they do still have a gut cavity. Examples of acoelomates include animals in the phylum Platyhelminthes, also known as flatworms. Animals with a true coelom are called eucoelomates (or coelomates) (Figure \(5\)). A true coelom arises entirely within the mesoderm germ layer and is lined by an epithelial membrane. This membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some free motion. Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates. A third group of triploblasts has a slightly different coelom derived partly from mesoderm and partly from endoderm, which is found between the two layers. Although still functional, these are considered false coeloms, and those animals are called pseudocoelomates. The phylum Nematoda (roundworms) is an example of a pseudocoelomate. True coelomates can be further characterized based on certain features of their early embryological development.
Embryonic Development of the Mouth
Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on differences in their early embryonic development. Protostomes include arthropods, mollusks, and annelids. Deuterostomes include more complex animals such as chordates but also some simple animals such as echinoderms. These two groups are separated based on which opening of the digestive cavity develops first: mouth or anus. The word protostome comes from the Greek word meaning “mouth first,” and deuterostome originates from the word meaning “mouth second” (in this case, the anus develops first). The mouth or anus develops from a structure called the blastopore (Figure \(6\)). The blastopore is the indentation formed during the initial stages of gastrulation. In later stages, a second opening forms, and these two openings will eventually give rise to the mouth and anus (Figure \(6\)). It has long been believed that the blastopore develops into the mouth of protostomes, with the second opening developing into the anus; the opposite is true for deuterostomes. Recent evidence has challenged this view of the development of the blastopore of protostomes, however, and the theory remains under debate.
Another distinction between protostomes and deuterostomes is the method of coelom formation, beginning from the gastrula stage. The coelom of most protostomes is formed through a process called schizocoely, meaning that during development, a solid mass of the mesoderm splits apart and forms the hollow opening of the coelom. Deuterostomes differ in that their coelom forms through a process called enterocoely. Here, the mesoderm develops as pouches that are pinched off from the endoderm tissue. These pouches eventually fuse to form the mesoderm, which then gives rise to the coelom.
The earliest distinction between protostomes and deuterostomes is the type of cleavage undergone by the zygote. Protostomes undergo spiral cleavage, meaning that the cells of one pole of the embryo are rotated, and thus misaligned, with respect to the cells of the opposite pole. This is due to the oblique angle of the cleavage. Deuterostomes undergo radial cleavage, where the cleavage axes are either parallel or perpendicular to the polar axis, resulting in the alignment of the cells between the two poles.
There is a second distinction between the types of cleavage in protostomes and deuterostomes. In addition to spiral cleavage, protostomes also undergo determinate cleavage. This means that even at this early stage, the developmental fate of each embryonic cell is already determined. A cell does not have the ability to develop into any cell type. In contrast, deuterostomes undergo indeterminate cleavage, in which cells are not yet pre-determined at this early stage to develop into specific cell types. These cells are referred to as undifferentiated cells. This characteristic of deuterostomes is reflected in the existence of familiar embryonic stem cells, which have the ability to develop into any cell type until their fate is programmed at a later developmental stage.
Evolution Connection: The Evolution of the Coelom
One of the first steps in the classification of animals is to examine the animal’s body. Studying the body parts tells us not only the roles of the organs in question but also how the species may have evolved. One such structure that is used in classification of animals is the coelom. A coelom is a body cavity that forms during early embryonic development. The coelom allows for compartmentalization of the body parts, so that different organ systems can evolve and nutrient transport is possible. Additionally, because the coelom is a fluid-filled cavity, it protects the organs from shock and compression. Simple animals, such as worms and jellyfish, do not have a coelom. All vertebrates have a coelom that helped them evolve complex organ systems.
Animals that do not have a coelom are called acoelomates. Flatworms and tapeworms are examples of acoelomates. They rely on passive diffusion for nutrient transport across their body. Additionally, the internal organs of acoelomates are not protected from crushing.
Animals that have a true coelom are called eucoelomates; all vertebrates are eucoelomates. The coelom evolves from the mesoderm during embryogenesis. The abdominal cavity contains the stomach, liver, gall bladder, and other digestive organs. Another category of invertebrates animals based on body cavity is pseudocoelomates. These animals have a pseudo-cavity that is not completely lined by mesoderm. Examples include nematode parasites and small worms. These animals are thought to have evolved from coelomates and may have lost their ability to form a coelom through genetic mutations. Thus, this step in early embryogenesis—the formation of the coelom—has had a large evolutionary impact on the various species of the animal kingdom.
Summary
Organisms in the animal kingdom are classified based on their body morphology and development. True animals are divided into those with radial versus bilateral symmetry. Generally, the simpler and often non-motile animals display radial symmetry. Animals with radial symmetry are also generally characterized by the development of two embryological germ layers, the endoderm and ectoderm, whereas animals with bilateral symmetry are generally characterized by the development of a third embryological germ layer, the mesoderm. Animals with three germ layers, called triploblasts, are further characterized by the presence or absence of an internal body cavity called a coelom. The presence of a coelom affords many advantages, and animals with a coelom may be termed true coelomates or pseudocoelomates, depending on which tissue gives rise to the coelom. Coelomates are further divided into one of two groups called protostomes and deuterostomes, based on a number of developmental characteristics, including differences in zygote cleavage and method of coelom formation.
Art Connections
Figure \(1\): Which of the following statements is false?
1. Eumetazoans have specialized tissues and parazoans don’t.
2. Lophotrochozoa and Ecdysozoa are both Bilataria.
3. Acoela and Cnidaria both possess radial symmetry.
4. Arthropods are more closely related to nematodes than they are to annelids.
Answer
C
Figure \(4\): Which of the following statements about diploblasts and triploblasts is false?
1. Animals that display radial symmetry are diploblasts.
2. Animals that display bilateral symmetry are triploblasts.
3. The endoderm gives rise to the lining of the digestive tract and the respiratory tract.
4. The mesoderm gives rise to the central nervous system.
Answer
D
Glossary
acoelomate
animal without a body cavity
bilateral symmetry
type of symmetry in which there is only one plane of symmetry, so the left and right halves of an animal are mirror images
blastopore
indentation formed during gastrulation, evident in the gastrula stage
coelom
lined body cavity
determinate cleavage
developmental tissue fate of each embryonic cell is already determined
deuterostome
blastopore develops into the anus, with the second opening developing into the mouth
diploblast
animal that develops from two germ layers
enterocoely
mesoderm of deuterostomes develops as pouches that are pinched off from endodermal tissue, cavity contained within the pouches becomes coelom
eucoelomate
animal with a body cavity completely lined with mesodermal tissue
indeterminate cleavage
early stage of development when germ cells or “stem cells” are not yet pre-determined to develop into specific cell types
protostome
blastopore develops into the mouth of protostomes, with the second opening developing into the anus
pseudocoelomate
animal with a body cavity located between the mesoderm and endoderm
radial cleavage
cleavage axes are parallel or perpendicular to the polar axis, resulting in the alignment of cells between the two poles
radial symmetry
type of symmetry with multiple planes of symmetry, with body parts (rays) arranged around a central disk
schizocoely
during development of protostomes, a solid mass of mesoderm splits apart and forms the hollow opening of the coelom
spiral cleavage
cells of one pole of the embryo are rotated or misaligned with respect to the cells of the opposite pole
triploblast
animal that develops from three germ layers | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/32%3A_Animal_Diversity_and_the_Evolution_of_Body_Plans/32.02%3A_Evolution_of_the_Animal_Body_Plan.txt |
Skills to Develop
• Explain the differences in animal body plans that support basic animal classification
• Compare and contrast the embryonic development of protostomes and deuterostomes
Scientists have developed a classification scheme that categorizes all members of the animal kingdom, although there are exceptions to most “rules” governing animal classification (Figure \(1\)). Animals are primarily classified according to morphological and developmental characteristics, such as a body plan. One of the most prominent features of the body plan of true animals is that they are morphologically symmetrical. This means that their distribution of body parts is balanced along an axis. Additional characteristics include the number of tissue layers formed during development, the presence or absence of an internal body cavity, and other features of embryological development, such as the origin of the mouth and anus.
Art Connection
Which of the following statements is false?
1. Eumetazoans have specialized tissues and parazoans don’t.
2. Lophotrochozoa and Ecdysozoa are both Bilataria.
3. Acoela and Cnidaria both possess radial symmetry.
4. Arthropods are more closely related to nematodes than they are to annelids.
Animal Characterization Based on Body Symmetry
At a very basic level of classification, true animals can be largely divided into three groups based on the type of symmetry of their body plan: radially symmetrical, bilaterally symmetrical, and asymmetrical. Asymmetry is a unique feature of Parazoa (Figure \(2\)). Only a few animal groups display radial symmetry. All types of symmetry are well suited to meet the unique demands of a particular animal’s lifestyle.
Radial symmetry is the arrangement of body parts around a central axis, as is seen in a drinking glass or pie. It results in animals having top and bottom surfaces but no left and right sides, or front or back. The two halves of a radially symmetrical animal may be described as the side with a mouth or “oral side,” and the side without a mouth (the “aboral side”). This form of symmetry marks the body plans of animals in the phyla Ctenophora and Cnidaria, including jellyfish and adult sea anemones (Figure 27.2.2). Radial symmetry equips these sea creatures (which may be sedentary or only capable of slow movement or floating) to experience the environment equally from all directions.
Bilateral symmetry involves the division of the animal through a sagittal plane, resulting in two mirror image, right and left halves, such as those of a butterfly (Figure \(2\)), crab, or human body. Animals with bilateral symmetry have a “head” and “tail” (anterior vs. posterior), front and back (dorsal vs. ventral), and right and left sides (Figure \(3\)). All true animals except those with radial symmetry are bilaterally symmetrical. The evolution of bilateral symmetry that allowed for the formation of anterior and posterior (head and tail) ends promoted a phenomenon called cephalization, which refers to the collection of an organized nervous system at the animal’s anterior end. In contrast to radial symmetry, which is best suited for stationary or limited-motion lifestyles, bilateral symmetry allows for streamlined and directional motion. In evolutionary terms, this simple form of symmetry promoted active mobility and increased sophistication of resource-seeking and predator-prey relationships.
Animals in the phylum Echinodermata (such as sea stars, sand dollars, and sea urchins) display radial symmetry as adults, but their larval stages exhibit bilateral symmetry. This is termed secondary radial symmetry. They are believed to have evolved from bilaterally symmetrical animals; thus, they are classified as bilaterally symmetrical.
Link to Learning
Watch this video to see a quick sketch of the different types of body symmetry.
Animal Characterization Based on Features of Embryological Development
Most animal species undergo a separation of tissues into germ layers during embryonic development. Recall that these germ layers are formed during gastrulation, and that they are predetermined to develop into the animal’s specialized tissues and organs. Animals develop either two or three embryonic germs layers (Figure \(4\)). The animals that display radial symmetry develop two germ layers, an inner layer (endoderm) and an outer layer (ectoderm). These animals are called diploblasts. Diploblasts have a non-living layer between the endoderm and ectoderm. More complex animals (those with bilateral symmetry) develop three tissue layers: an inner layer (endoderm), an outer layer (ectoderm), and a middle layer (mesoderm). Animals with three tissue layers are called triploblasts.
Art Connection
Which of the following statements about diploblasts and triploblasts is false?
1. Animals that display radial symmetry are diploblasts.
2. Animals that display bilateral symmetry are triploblasts.
3. The endoderm gives rise to the lining of the digestive tract and the respiratory tract.
4. The mesoderm gives rise to the central nervous system.
Each of the three germ layers is programmed to give rise to particular body tissues and organs. The endoderm gives rise to the lining of the digestive tract (including the stomach, intestines, liver, and pancreas), as well as to the lining of the trachea, bronchi, and lungs of the respiratory tract, along with a few other structures. The ectoderm develops into the outer epithelial covering of the body surface, the central nervous system, and a few other structures. The mesoderm is the third germ layer; it forms between the endoderm and ectoderm in triploblasts. This germ layer gives rise to all muscle tissues (including the cardiac tissues and muscles of the intestines), connective tissues such as the skeleton and blood cells, and most other visceral organs such as the kidneys and the spleen.
Presence or Absence of a Coelom
Further subdivision of animals with three germ layers (triploblasts) results in the separation of animals that may develop an internal body cavity derived from mesoderm, called a coelom, and those that do not. This epithelial cell-lined coelomic cavity represents a space, usually filled with fluid, which lies between the visceral organs and the body wall. It houses many organs such as the digestive system, kidneys, reproductive organs, and heart, and contains the circulatory system. In some animals, such as mammals, the part of the coelom called the pleural cavity provides space for the lungs to expand during breathing. The evolution of the coelom is associated with many functional advantages. Primarily, the coelom provides cushioning and shock absorption for the major organ systems. Organs housed within the coelom can grow and move freely, which promotes optimal organ development and placement. The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility, promoting improved animal motility.
Triploblasts that do not develop a coelom are called acoelomates, and their mesoderm region is completely filled with tissue, although they do still have a gut cavity. Examples of acoelomates include animals in the phylum Platyhelminthes, also known as flatworms. Animals with a true coelom are called eucoelomates (or coelomates) (Figure \(5\)). A true coelom arises entirely within the mesoderm germ layer and is lined by an epithelial membrane. This membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some free motion. Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates. A third group of triploblasts has a slightly different coelom derived partly from mesoderm and partly from endoderm, which is found between the two layers. Although still functional, these are considered false coeloms, and those animals are called pseudocoelomates. The phylum Nematoda (roundworms) is an example of a pseudocoelomate. True coelomates can be further characterized based on certain features of their early embryological development.
Embryonic Development of the Mouth
Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on differences in their early embryonic development. Protostomes include arthropods, mollusks, and annelids. Deuterostomes include more complex animals such as chordates but also some simple animals such as echinoderms. These two groups are separated based on which opening of the digestive cavity develops first: mouth or anus. The word protostome comes from the Greek word meaning “mouth first,” and deuterostome originates from the word meaning “mouth second” (in this case, the anus develops first). The mouth or anus develops from a structure called the blastopore (Figure \(6\)). The blastopore is the indentation formed during the initial stages of gastrulation. In later stages, a second opening forms, and these two openings will eventually give rise to the mouth and anus (Figure \(6\)). It has long been believed that the blastopore develops into the mouth of protostomes, with the second opening developing into the anus; the opposite is true for deuterostomes. Recent evidence has challenged this view of the development of the blastopore of protostomes, however, and the theory remains under debate.
Another distinction between protostomes and deuterostomes is the method of coelom formation, beginning from the gastrula stage. The coelom of most protostomes is formed through a process called schizocoely, meaning that during development, a solid mass of the mesoderm splits apart and forms the hollow opening of the coelom. Deuterostomes differ in that their coelom forms through a process called enterocoely. Here, the mesoderm develops as pouches that are pinched off from the endoderm tissue. These pouches eventually fuse to form the mesoderm, which then gives rise to the coelom.
The earliest distinction between protostomes and deuterostomes is the type of cleavage undergone by the zygote. Protostomes undergo spiral cleavage, meaning that the cells of one pole of the embryo are rotated, and thus misaligned, with respect to the cells of the opposite pole. This is due to the oblique angle of the cleavage. Deuterostomes undergo radial cleavage, where the cleavage axes are either parallel or perpendicular to the polar axis, resulting in the alignment of the cells between the two poles.
There is a second distinction between the types of cleavage in protostomes and deuterostomes. In addition to spiral cleavage, protostomes also undergo determinate cleavage. This means that even at this early stage, the developmental fate of each embryonic cell is already determined. A cell does not have the ability to develop into any cell type. In contrast, deuterostomes undergo indeterminate cleavage, in which cells are not yet pre-determined at this early stage to develop into specific cell types. These cells are referred to as undifferentiated cells. This characteristic of deuterostomes is reflected in the existence of familiar embryonic stem cells, which have the ability to develop into any cell type until their fate is programmed at a later developmental stage.
Evolution Connection: The Evolution of the Coelom
One of the first steps in the classification of animals is to examine the animal’s body. Studying the body parts tells us not only the roles of the organs in question but also how the species may have evolved. One such structure that is used in classification of animals is the coelom. A coelom is a body cavity that forms during early embryonic development. The coelom allows for compartmentalization of the body parts, so that different organ systems can evolve and nutrient transport is possible. Additionally, because the coelom is a fluid-filled cavity, it protects the organs from shock and compression. Simple animals, such as worms and jellyfish, do not have a coelom. All vertebrates have a coelom that helped them evolve complex organ systems.
Animals that do not have a coelom are called acoelomates. Flatworms and tapeworms are examples of acoelomates. They rely on passive diffusion for nutrient transport across their body. Additionally, the internal organs of acoelomates are not protected from crushing.
Animals that have a true coelom are called eucoelomates; all vertebrates are eucoelomates. The coelom evolves from the mesoderm during embryogenesis. The abdominal cavity contains the stomach, liver, gall bladder, and other digestive organs. Another category of invertebrates animals based on body cavity is pseudocoelomates. These animals have a pseudo-cavity that is not completely lined by mesoderm. Examples include nematode parasites and small worms. These animals are thought to have evolved from coelomates and may have lost their ability to form a coelom through genetic mutations. Thus, this step in early embryogenesis—the formation of the coelom—has had a large evolutionary impact on the various species of the animal kingdom.
Summary
Organisms in the animal kingdom are classified based on their body morphology and development. True animals are divided into those with radial versus bilateral symmetry. Generally, the simpler and often non-motile animals display radial symmetry. Animals with radial symmetry are also generally characterized by the development of two embryological germ layers, the endoderm and ectoderm, whereas animals with bilateral symmetry are generally characterized by the development of a third embryological germ layer, the mesoderm. Animals with three germ layers, called triploblasts, are further characterized by the presence or absence of an internal body cavity called a coelom. The presence of a coelom affords many advantages, and animals with a coelom may be termed true coelomates or pseudocoelomates, depending on which tissue gives rise to the coelom. Coelomates are further divided into one of two groups called protostomes and deuterostomes, based on a number of developmental characteristics, including differences in zygote cleavage and method of coelom formation.
Art Connections
Figure \(1\): Which of the following statements is false?
1. Eumetazoans have specialized tissues and parazoans don’t.
2. Lophotrochozoa and Ecdysozoa are both Bilataria.
3. Acoela and Cnidaria both possess radial symmetry.
4. Arthropods are more closely related to nematodes than they are to annelids.
Answer
C
Figure \(4\): Which of the following statements about diploblasts and triploblasts is false?
1. Animals that display radial symmetry are diploblasts.
2. Animals that display bilateral symmetry are triploblasts.
3. The endoderm gives rise to the lining of the digestive tract and the respiratory tract.
4. The mesoderm gives rise to the central nervous system.
Answer
D
Glossary
acoelomate
animal without a body cavity
bilateral symmetry
type of symmetry in which there is only one plane of symmetry, so the left and right halves of an animal are mirror images
blastopore
indentation formed during gastrulation, evident in the gastrula stage
coelom
lined body cavity
determinate cleavage
developmental tissue fate of each embryonic cell is already determined
deuterostome
blastopore develops into the anus, with the second opening developing into the mouth
diploblast
animal that develops from two germ layers
enterocoely
mesoderm of deuterostomes develops as pouches that are pinched off from endodermal tissue, cavity contained within the pouches becomes coelom
eucoelomate
animal with a body cavity completely lined with mesodermal tissue
indeterminate cleavage
early stage of development when germ cells or “stem cells” are not yet pre-determined to develop into specific cell types
protostome
blastopore develops into the mouth of protostomes, with the second opening developing into the anus
pseudocoelomate
animal with a body cavity located between the mesoderm and endoderm
radial cleavage
cleavage axes are parallel or perpendicular to the polar axis, resulting in the alignment of cells between the two poles
radial symmetry
type of symmetry with multiple planes of symmetry, with body parts (rays) arranged around a central disk
schizocoely
during development of protostomes, a solid mass of mesoderm splits apart and forms the hollow opening of the coelom
spiral cleavage
cells of one pole of the embryo are rotated or misaligned with respect to the cells of the opposite pole
triploblast
animal that develops from three germ layers
32.03: Animal Phylogeny
Skills to Develop
• Interpret the metazoan phylogenetic tree
• Describe the types of data that scientists use to construct and revise animal phylogeny
• List some of the relationships within the modern phylogenetic tree that have been discovered as a result of modern molecular data
Biologists strive to understand the evolutionary history and relationships of members of the animal kingdom, and all of life, for that matter. The study of phylogeny aims to determine the evolutionary relationships between phyla. Currently, most biologists divide the animal kingdom into 35 to 40 phyla. Scientists develop phylogenetic trees, which serve as hypotheses about which species have evolved from which ancestors
Recall that until recently, only morphological characteristics and the fossil record were used to determine phylogenetic relationships among animals. Scientific understanding of the distinctions and hierarchies between anatomical characteristics provided much of this knowledge. Used alone, however, this information can be misleading. Morphological characteristics may evolve multiple times, and independently, through evolutionary history. Analogous characteristics may appear similar between animals, but their underlying evolution may be very different. With the advancement of molecular technologies, modern phylogenetics is now informed by genetic and molecular analyses, in addition to traditional morphological and fossil data. With a growing understanding of genetics, the animal evolutionary tree has changed substantially and continues to change as new DNA and RNA analyses are performed on additional animal species.
Constructing an Animal Phylogenetic Tree
The current understanding of evolutionary relationships between animal, or Metazoa, phyla begins with the distinction between “true” animals with true differentiated tissues, called Eumetazoa, and animal phyla that do not have true differentiated tissues (such as the sponges), called Parazoa. Both Parazoa and Eumetazoa evolved from a common ancestral organism that resembles the modern-day protists called choanoflagellates. These protist cells strongly resemble the sponge choanocyte cells today (Figure \(1\)).
Eumetazoa are subdivided into radially symmetrical animals and bilaterally symmetrical animals, and are thus classified into clade Bilateria or Radiata, respectively. As mentioned earlier, the cnidarians and ctenophores are animal phyla with true radial symmetry. All other Eumetazoa are members of the Bilateria clade. The bilaterally symmetrical animals are further divided into deuterostomes (including chordates and echinoderms) and two distinct clades of protostomes (including ecdysozoans and lophotrochozoans) (Figure \(2\)). Ecdysozoa includes nematodes and arthropods; they are so named for a commonly found characteristic among the group: exoskeletal molting (termed ecdysis). Lophotrochozoa is named for two structural features, each common to certain phyla within the clade. Some lophotrochozoan phyla are characterized by a larval stage called trochophore larvae, and other phyla are characterized by the presence of a feeding structure called a lophophore.
Link to Learning
Explore an interactive tree of life here. Zoom and click to learn more about the organisms and their evolutionary relationships.
Modern Advances in Phylogenetic Understanding Come from Molecular Analyses
The phylogenetic groupings are continually being debated and refined by evolutionary biologists. Each year, new evidence emerges that further alters the relationships described by a phylogenetic tree diagram.
Link to Learning
Watch the following video to learn how biologists use genetic data to determine relationships among organisms.
Nucleic acid and protein analyses have greatly informed the modern phylogenetic animal tree. These data come from a variety of molecular sources, such as mitochondrial DNA, nuclear DNA, ribosomal RNA (rRNA), and certain cellular proteins. Many evolutionary relationships in the modern tree have only recently been determined due to molecular evidence. For example, a previously classified group of animals called lophophorates, which included brachiopods and bryozoans, were long-thought to be primitive deuterostomes. Extensive molecular analysis using rRNA data found these animals to be protostomes, more closely related to annelids and mollusks. This discovery allowed for the distinction of the protostome clade, the lophotrochozoans. Molecular data have also shed light on some differences within the lophotrochozoan group, and some scientists believe that the phyla Platyhelminthes and Rotifera within this group should actually belong to their own group of protostomes termed Platyzoa.
Molecular research similar to the discoveries that brought about the distinction of the lophotrochozoan clade has also revealed a dramatic rearrangement of the relationships between mollusks, annelids, arthropods, and nematodes, and a new ecdysozoan clade was formed. Due to morphological similarities in their segmented body types, annelids and arthropods were once thought to be closely related. However, molecular evidence has revealed that arthropods are actually more closely related to nematodes, now comprising the ecdysozoan clade, and annelids are more closely related to mollusks, brachiopods, and other phyla in the lophotrochozoan clade. These two clades now make up the protostomes.
Another change to former phylogenetic groupings because of molecular analyses includes the emergence of an entirely new phylum of worm called Acoelomorpha. These acoel flatworms were long thought to belong to the phylum Platyhelminthes because of their similar “flatworm” morphology. However, molecular analyses revealed this to be a false relationship and originally suggested that acoels represented living species of some of the earliest divergent bilaterians. More recent research into the acoelomorphs has called this hypothesis into question and suggested a closer relationship with deuterostomes. The placement of this new phylum remains disputed, but scientists agree that with sufficient molecular data, their true phylogeny will be determined.
Summary
Scientists are interested in the evolutionary history of animals and the evolutionary relationships among them. There are three main sources of data that scientists use to create phylogenetic evolutionary tree diagrams that illustrate such relationships: morphological information (which includes developmental morphologies), fossil record data, and, most recently, molecular data. The details of the modern phylogenetic tree change frequently as new data are gathered, and molecular data has recently contributed to many substantial modifications of the understanding of relationships between animal phyla.
Glossary
Ecdysozoa
clade of protostomes that exhibit exoskeletal molting (ecdysis)
Eumetazoa
group of animals with true differentiated tissues
Lophotrochozoa
clade of protostomes that exhibit a trochophore larvae stage or a lophophore feeding structure
Metazoa
group containing all animals
Parazoa
group of animals without true differentiated tissues | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/32%3A_Animal_Diversity_and_the_Evolution_of_Body_Plans/32.03%3A_Animal_Phylogeny/32.3.01%3A_Animal_Phylogeny.txt |
Skills to Develop
• Describe the organizational features of the simplest multicellular organisms
• Explain the various body forms and bodily functions of sponges
The invertebrates, or invertebrata, are animals that do not contain bony structures, such as the cranium and vertebrae. The simplest of all the invertebrates are the Parazoans, which include only the phylum Porifera: the sponges (Figure \(1\)). Parazoans (“beside animals”) do not display tissue-level organization, although they do have specialized cells that perform specific functions. Sponge larvae are able to swim; however, adults are non-motile and spend their life attached to a substratum. Since water is vital to sponges for excretion, feeding, and gas exchange, their body structure facilitates the movement of water through the sponge. Structures such as canals, chambers, and cavities enable water to move through the sponge to nearly all body cells.
Morphology of Sponges
The morphology of the simplest sponges takes the shape of a cylinder with a large central cavity, the spongocoel, occupying the inside of the cylinder. Water can enter into the spongocoel from numerous pores in the body wall. Water entering the spongocoel is extruded via a large common opening called the osculum. However, sponges exhibit a range of diversity in body forms, including variations in the size of the spongocoel, the number of osculi, and where the cells that filter food from the water are located.
While sponges (excluding the hexactinellids) do not exhibit tissue-layer organization, they do have different cell types that perform distinct functions. Pinacocytes, which are epithelial-like cells, form the outermost layer of sponges and enclose a jelly-like substance called mesohyl. Mesohyl is an extracellular matrix consisting of a collagen-like gel with suspended cells that perform various functions. The gel-like consistency of mesohyl acts like an endoskeleton and maintains the tubular morphology of sponges. In addition to the osculum, sponges have multiple pores called ostia on their bodies that allow water to enter the sponge. In some sponges, ostia are formed by porocytes, single tube-shaped cells that act as valves to regulate the flow of water into the spongocoel. In other sponges, ostia are formed by folds in the body wall of the sponge.
Choanocytes (“collar cells”) are present at various locations, depending on the type of sponge, but they always line the inner portions of some space through which water flows (the spongocoel in simple sponges, canals within the body wall in more complex sponges, and chambers scattered throughout the body in the most complex sponges). Whereas pinacocytes line the outside of the sponge, choanocytes tend to line certain inner portions of the sponge body that surround the mesohyl. The structure of a choanocyte is critical to its function, which is to generate a water current through the sponge and to trap and ingest food particles by phagocytosis. Note the similarity in appearance between the sponge choanocyte and choanoflagellates (Protista). This similarity suggests that sponges and choanoflagellates are closely related and likely share a recent common ancestry. The cell body is embedded in mesohyl and contains all organelles required for normal cell function, but protruding into the “open space” inside of the sponge is a mesh-like collar composed of microvilli with a single flagellum in the center of the column. The cumulative effect of the flagella from all choanocytes aids the movement of water through the sponge: drawing water into the sponge through the numerous ostia, into the spaces lined by choanocytes, and eventually out through the osculum (or osculi). In the meantime, food particles, including waterborne bacteria and algae, are trapped by the sieve-like collar of the choanocytes, slide down into the body of the cell, are ingested by phagocytosis, and become encased in a food vacuole. Lastly, choanocytes will differentiate into sperm for sexual reproduction, where they will become dislodged from the mesohyl and leave the sponge with expelled water through the osculum.
The second crucial cells in sponges are called amoebocytes (or archaeocytes), named for the fact that they move throughout the mesohyl in an amoeba-like fashion. Amoebocytes have a variety of functions: delivering nutrients from choanocytes to other cells within the sponge, giving rise to eggs for sexual reproduction (which remain in the mesohyl), delivering phagocytized sperm from choanocytes to eggs, and differentiating into more-specific cell types. Some of these more-specific cell types include collencytes and lophocytes, which produce the collagen-like protein to maintain the mesohyl, sclerocytes, which produce spicules in some sponges, and spongocytes, which produce the protein spongin in the majority of sponges. These cells produce collagen to maintain the consistency of the mesohyl. The different cell types in sponges are shown in Figure \(2\).
Exercise \(1\)
Which of the following statements is false?
1. Choanocytes have flagella that propel water through the body.
2. Pinacocytes can transform into any cell type.
3. Lophocytes secrete collagen.
4. Porocytes control the flow of water through pores in the sponge body.
Answer
B
In some sponges, sclerocytes secrete small spicules into the mesohyl, which are composed of either calcium carbonate or silica, depending on the type of sponge. These spicules serve to provide additional stiffness to the body of the sponge. Additionally, spicules, when present externally, may ward off predators. Another type of protein, spongin, may also be present in the mesohyl of some sponges.
The presence and composition of spicules/spongin are the differentiating characteristics of the three classes of sponges (Figure \(3\)): Class Calcarea contains calcium carbonate spicules and no spongin, class Hexactinellida contains six-rayed siliceous spicules and no spongin, and class Demospongia contains spongin and may or may not have spicules; if present, those spicules are siliceous. Spicules are most conspicuously present in class Hexactinellida, the order consisting of glass sponges. Some of the spicules may attain giant proportions (in relation to the typical size range of glass sponges of 3 to 10 mm) as seen in Monorhaphis chuni, which grows up to 3 m long.
Physiological Processes in Sponges
Sponges, despite being simple organisms, regulate their different physiological processes through a variety of mechanisms. These processes regulate their metabolism, reproduction, and locomotion.
Digestion
Sponges lack complex digestive, respiratory, circulatory, reproductive, and nervous systems. Their food is trapped when water passes through the ostia and out through the osculum. Bacteria smaller than 0.5 microns in size are trapped by choanocytes, which are the principal cells engaged in nutrition, and are ingested by phagocytosis. Particles that are larger than the ostia may be phagocytized by pinacocytes. In some sponges, amoebocytes transport food from cells that have ingested food particles to those that do not. For this type of digestion, in which food particles are digested within individual cells, the sponge draws water through diffusion. The limit of this type of digestion is that food particles must be smaller than individual cells.
All other major body functions in the sponge (gas exchange, circulation, excretion) are performed by diffusion between the cells that line the openings within the sponge and the water that is passing through those openings. All cell types within the sponge obtain oxygen from water through diffusion. Likewise, carbon dioxide is released into seawater by diffusion. In addition, nitrogenous waste produced as a byproduct of protein metabolism is excreted via diffusion by individual cells into the water as it passes through the sponge.
Reproduction
Sponges reproduce by sexual as well as asexual methods. The typical means of asexual reproduction is either fragmentation (where a piece of the sponge breaks off, settles on a new substrate, and develops into a new individual) or budding (a genetically identical outgrowth grows from the parent and eventually detaches or remains attached to form a colony). An atypical type of asexual reproduction is found only in freshwater sponges and occurs through the formation of gemmules. Gemmules are environmentally resistant structures produced by adult sponges wherein the typical sponge morphology is inverted. In gemmules, an inner layer of amoebocytes is surrounded by a layer of collagen (spongin) that may be reinforced by spicules. The collagen that is normally found in the mesohyl becomes the outer protective layer. In freshwater sponges, gemmules may survive hostile environmental conditions like changes in temperature and serve to recolonize the habitat once environmental conditions stabilize. Gemmules are capable of attaching to a substratum and generating a new sponge. Since gemmules can withstand harsh environments, are resistant to desiccation, and remain dormant for long periods, they are an excellent means of colonization for a sessile organism.
Sexual reproduction in sponges occurs when gametes are generated. Sponges are monoecious (hermaphroditic), which means that one individual can produce both gametes (eggs and sperm) simultaneously. In some sponges, production of gametes may occur throughout the year, whereas other sponges may show sexual cycles depending upon water temperature. Sponges may also become sequentially hermaphroditic, producing oocytes first and spermatozoa later. Oocytes arise by the differentiation of amoebocytes and are retained within the spongocoel, whereas spermatozoa result from the differentiation of choanocytes and are ejected via the osculum. Ejection of spermatozoa may be a timed and coordinated event, as seen in certain species. Spermatozoa carried along by water currents can fertilize the oocytes borne in the mesohyl of other sponges. Early larval development occurs within the sponge, and free-swimming larvae are then released via the osculum.
Locomotion
Sponges are generally sessile as adults and spend their lives attached to a fixed substratum. They do not show movement over large distances like other free-swimming marine invertebrates. However, sponge cells are capable of creeping along substrata via organizational plasticity. Under experimental conditions, researchers have shown that sponge cells spread on a physical support demonstrate a leading edge for directed movement. It has been speculated that this localized creeping movement may help sponges adjust to microenvironments near the point of attachment. It must be noted, however, that this pattern of movement has been documented in laboratories, but it remains to be observed in natural sponge habitats.
Summary
Animals included in phylum Porifera are Parazoans because they do not show the formation of true tissues (except in class Hexactinellida). These organisms show very simple organization, with a rudimentary endoskeleton. Sponges have multiple cell types that are geared toward executing various metabolic functions. Although these animals are very simple, they perform several complex physiological functions.
Glossary
amoebocyte
sponge cell with multiple functions, including nutrient delivery, egg formation, sperm delivery, and cell differentiation
choanocyte
(also, collar cell) sponge cell that functions to generate a water current and to trap and ingest food particles via phagocytosis
gemmule
structure produced by asexual reproduction in freshwater sponges where the morphology is inverted
invertebrata
(also, invertebrates) category of animals that do not possess a cranium or vertebral column
mesohyl
collagen-like gel containing suspended cells that perform various functions in the sponge
osculum
large opening in the sponge’s body through which water leaves
ostium
pore present on the sponge’s body through which water enters
pinacocyte
epithelial-like cell that forms the outermost layer of sponges and encloses a jelly-like substance called mesohyl
Porifera
phylum of animals with no true tissues, but a porous body with rudimentary endoskeleton
sclerocyte
cell that secretes silica spicules into the mesohyl
spicule
structure made of silica or calcium carbonate that provides structural support for sponges
spongocoel
central cavity within the body of some sponges | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/32%3A_Animal_Diversity_and_the_Evolution_of_Body_Plans/32.04%3A_Parazoa-_Animals_that_Lack_Specialized_Tissues.txt |
Phylogenetic trees are constructed according to the evolutionary relationships that exist between organisms based on homologous traits.
Learning Objectives
• Describe the information needed to construct a phylogenetic tree of animals
Key Points
• Phylogenetic trees are constructed using various data derived from studies on homologous traits, analagous traits, and molecular evidence that can be used to establish relationships using polymeric molecules ( DNA, RNA, and proteins ).
• Evolutionary relationships between animal phyla, or Metazoa, are based on the the presence or absence of differentiated tissues, referred to as Eumetazoa or Parazoa, respectively.
• Eumetazoa can be further classified into categories that are based on whether they have radial or bilateral symmetry, referred to as Radiata or Bilateria, respectively.
Key Terms
• orthologous: having been separated by a speciation event
• homoplasy: a correspondence between the parts or organs of different species acquired as the result of parallel evolution or convergence
Constructing an Animal Phylogenetic Tree
Evolutionary trees, or phylogeny, is the formal study of organisms and their evolutionary history with respect to each other. Phylogenetic trees are most-commonly used to depict the relationships that exist between species. In particular, they clarify whether certain traits are homologous (found in the common ancestor as a result of divergent evolution) or homoplasy (sometimes referred to as analogous: a character that is not found in a common ancestor, but whose function developed independently in two or more organisms through convergent evolution). Evolutionary trees are diagrams that show various biological species and their evolutionary relationships. They consist of branches that flow from lower forms of life to the higher forms of life.
Evolutionary trees differ from taxonomy which is an ordered division of organisms into categories based on a set of characteristics used to assess similarities and differences. Evolutionary trees involve biological classification and use morphology to show relationships. Phylogeny is evolutionary history shown by the relationships found when comparing polymeric molecules such as RNA, DNA, or proteins of various organisms. The evolutionary pathway is analyzed by the sequence similarity of these polymeric molecules. This is based on the assumption that the similarities of sequence result from having fewer evolutionary divergences than others. The evolutionary tree is constructed by aligning the sequences; the length of the branch is proportional to the amount of amino acid differences between the sequences.
Phylogenetic systematics informs the construction of phylogenetic trees based on shared characters. Comparing nucleic acids or other molecules to infer relationships is a valuable tool for tracing an organism’s evolutionary history. The ability of molecular trees to encompass both short and long periods of time is hinged on the ability of genes to evolve at different rates, even in the same evolutionary lineage. For example, the DNA that codes for rRNA changes relatively slowly, so comparisons of DNA sequences in these genes are useful for investigating relationships between taxa that diverged a long time ago. Interestingly, 99% of the genes in humans and mice are detectably orthologous, and 50% of our genes are orthologous with those of yeast. The hemoglobin B genes in humans and in mice are orthologous. These genes serve similar functions, but their sequences have diverged since the time that humans and mice had a common ancestor.
Evolutionary pathways relating the members of a family of proteins may be deduced by examination of sequence similarity. This approach is based on the notion that sequences that are more similar to one another have had less evolutionary time to diverge than have sequences that are less similar. Evolutionary trees are used today for DNA hybridization, which determines the percentage difference of genetic material between two similar species. If there is a high resemblance of DNA between the two species, then the species are closely related. If only a small percentage is identical, then they are distantly related.
Animal Phyla
The current understanding of evolutionary relationships between animal, or Metazoa, phyla begins with the distinction between “true” animals with true differentiated tissues, called Eumetazoa, and animal phyla that do not have true differentiated tissues (such as the sponges), called Parazoa. Both Parazoa and Eumetazoa evolved from a common ancestral organism that resembles the modern-day protists called choanoflagellates. These protist cells strongly resemble sponge choanocyte cells.
Eumetazoa are subdivided into radially-symmetrical animals and bilaterally-symmetrical animals and are classified into clade Radiata or Bilateria, respectively. The cnidarians and ctenophores are animal phyla with true radial symmetry. All other Eumetazoa are members of the Bilateria clade. The bilaterally-symmetrical animals are further divided into deuterostomes (including chordates and echinoderms) and two distinct clades of protostomes (including ecdysozoans and lophotrochozoans). Ecdysozoa includes nematodes and arthropods; named for a commonly-found characteristic among the group: exoskeletal molting (termed ecdysis). Lophotrochozoa is named for two structural features, each common to certain phyla within the clade. Some lophotrochozoan phyla are characterized by a larval stage called trochophore larvae, and other phyla are characterized by the presence of a feeding structure called a lophophore.
32.05: Eumetazoa- Animals with True Tissues
Cnidarians are diploblastic, have organized tissue, undergo extracellular digestion, and use cnidocytes for protection and to capture prey.
Learning Objectives
• Describe the fundamental anatomy of a Cnidarian
Key Points
• Cnidarians have two distinct morphological body plans known as polyp, which are sessile as adults, and medusa, which are mobile; some species exhibit both body plans in their lifecycle.
• All cnidarians have two membrane layers in the body: the epidermis and the gastrodermis; between both layers they have the mesoglea, which is a connective layer.
• Cnidarians carry out extracellular digestion, where enzymes break down the food particles and cells lining the gastrovascular cavity absorb the nutrients.
• Cnidarians have an incomplete digestive system with only one opening; the gastrovascular cavity serves as both a mouth and an anus.
• The nervous system of cnidarians, responsible for tentacle movement, drawing of captured prey to the mouth, digestion of food, and expulsion of waste, is composed of nerve cells scattered across the body.
• Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa make up the four different classes of Cnidarians.
Key Terms
• diploblastic: having two embryonic germ layers (the ectoderm and the endoderm)
• cnidocyte: a capsule, in certain cnidarians, containing a barbed, threadlike tube that delivers a paralyzing sting
Introduction to Phylum Cnidaria
Phylum Cnidaria includes animals that show radial or biradial symmetry and are diploblastic: they develop from two embryonic layers. Nearly all (about 99 percent) cnidarians are marine species.
Cnidarians contain specialized cells known as cnidocytes (“stinging cells”), which contain organelles called nematocysts (stingers). These cells are present around the mouth and tentacles, serving to immobilize prey with toxins contained within the cells. Nematocysts contain coiled threads that may bear barbs. The outer wall of the cell has hairlike projections called cnidocils, which are sensitive to touch. When touched, the cells are known to fire coiled threads that can either penetrate the flesh of the prey or predators of cnidarians, or ensnare it. These coiled threads release toxins into the target that can often immobilize prey or scare away predators ().
Animals in this phylum display two distinct morphological body plans: polyp or “stalk” and medusa or “bell”. An example of the polyp form is Hydra spp.; perhaps the most well-known medusoid animals are the jellies (jellyfish). Polyp forms are sessile as adults, with a single opening to the digestive system (the mouth) facing up with tentacles surrounding it. Medusa forms are motile, with the mouth and tentacles hanging down from an umbrella-shaped bell.
Some cnidarians are polymorphic, having two body plans during their life cycle. An example is the colonial hydroid called an Obelia. The sessile polyp form has, in fact, two types of polyps. The first is the gastrozooid, which is adapted for capturing prey and feeding; the other type of polyp is the gonozooid, adapted for the asexual budding of medusa. When the reproductive buds mature, they break off and become free-swimming medusa, which are either male or female (dioecious). The male medusa makes sperm, whereas the female medusa makes eggs. After fertilization, the zygote develops into a blastula and then into a planula larva. The larva is free swimming for a while, but eventually attaches and a new colonial reproductive polyp is formed.
All cnidarians show the presence of two membrane layers in the body that are derived from the endoderm and ectoderm of the embryo. The outer layer (from ectoderm) is called the epidermis and lines the outside of the animal, whereas the inner layer (from endoderm) is called the gastrodermis and lines the digestive cavity. Between these two membrane layers is a non-living, jelly-like mesoglea connective layer. In terms of cellular complexity, cnidarians show the presence of differentiated cell types in each tissue layer: nerve cells, contractile epithelial cells, enzyme-secreting cells, and nutrient-absorbing cells, as well as the presence of intercellular connections. However, the development of organs or organ systems is not advanced in this phylum.
The nervous system is primitive, with nerve cells scattered across the body. This nerve net may show the presence of groups of cells in the form of nerve plexi (singular: plexus) or nerve cords. The nerve cells show mixed characteristics of motor as well as sensory neurons. The predominant signaling molecules in these primitive nervous systems are chemical peptides, which perform both excitatory and inhibitory functions. Despite the simplicity of the nervous system, it coordinates the movement of tentacles, the drawing of captured prey to the mouth, the digestion of food, and the expulsion of waste.
The cnidarians perform extracellular digestion in which the food is taken into the gastrovascular cavity, enzymes are secreted into the cavity, and the cells lining the cavity absorb nutrients. The gastrovascular cavity has only one opening that serves as both a mouth and an anus; this is termed an incomplete digestive system. Cnidarian cells exchange oxygen and carbon dioxide by diffusion between cells in the epidermis with water in the environment, and between cells in the gastrodermis with water in the gastrovascular cavity. The lack of a circulatory system to move dissolved gases limits the thickness of the body wall, necessitating a non-living mesoglea between the layers. There is no excretory system or organs; nitrogenous wastes simply diffuse from the cells into the water outside the animal or in the gastrovascular cavity. There is also no circulatory system, so nutrients must move from the cells that absorb them in the lining of the gastrovascular cavity through the mesoglea to other cells.
The phylum Cnidaria contains about 10,000 described species divided into four classes: Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa. The anthozoans, the sea anemones and corals, are all sessile species, whereas the scyphozoans (jellyfish) and cubozoans (box jellies) are swimming forms. The hydrozoans contain sessile forms and swimming colonial forms like the Portuguese Man O’ War. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/32%3A_Animal_Diversity_and_the_Evolution_of_Body_Plans/32.05%3A_Eumetazoa-_Animals_with_True_Tissues/32.5A%3A_Phylum_Cnidaria.txt |
Members of the class Anthozoa display only polyp morphology and have cnidocyte-covered tentacles around their mouth opening.
Learning Objectives
• Identify the adaptive features of anthozoa
Key Points
• Anthozoans include sea anemones, sea pens, and corals.
• The pharynx of anthozoans (ingesting as well as egesting food) leads to the gastrovascular cavity, which is divided by mesenteries.
• In Anthozoans, gametes are produced by the polyp; if they fuse, they will give rise to a free-swimming planula larva, which will become sessile once it finds an optimal substrate.
• Sea anemonies and coral are examples of anthozoans that form unique mutualistic relationships with other animal species; both sea anemonies and coral benefit from food availability provided by their partners.
Key Terms
• mesentery: in invertebrates, it describes any tissue that divides the body cavity into partitions
• cnidocyte: a capsule, in certain cnidarians, containing a barbed, threadlike tube that delivers a paralyzing sting
• hermatypic: of a coral that is a species that builds coral reefs
Class Anthozoa
The class Anthozoa includes all cnidarians that exhibit a polyp body plan only; in other words, there is no medusa stage within their life cycle. Examples include sea anemones, sea pens, and corals, with an estimated number of 6,100 described species. Sea anemones are usually brightly colored and can attain a size of 1.8 to 10 cm in diameter. These animals are usually cylindrical in shape and are attached to a substrate.
The mouth of a sea anemone is surrounded by tentacles that bear cnidocytes. They have slit-like mouth openings and a pharynx, which is the muscular part of the digestive system that serves to ingest as well as egest food. It may extend for up to two-thirds the length of the body before opening into the gastrovascular cavity. This cavity is divided into several chambers by longitudinal septa called mesenteries. Each mesentery consists of one ectodermal and one endodermal cell layer with the mesoglea sandwiched in between. Mesenteries do not divide the gastrovascular cavity completely; the smaller cavities coalesce at the pharyngeal opening. The adaptive benefit of the mesenteries appears to be an increase in surface area for absorption of nutrients and gas exchange.
Sea anemones feed on small fish and shrimp, usually by immobilizing their prey using the cnidocytes. Some sea anemones establish a mutualistic relationship with hermit crabs by attaching to the crab’s shell. In this relationship, the anemone gets food particles from prey caught by the crab, while the crab is protected from the predators by the stinging cells of the anemone. Anemone fish, or clownfish, are able to live in the anemone since they are immune to the toxins contained within the nematocysts. Another type of anthozoan that forms an important mutualistic relationship is reef building coral. These hermatypic corals rely on a symbiotic relationship with zooxanthellae. The coral gains photosynthetic capability, while the zooxanthellae benefit by using nitrogenous waste and carbon dioxide produced by the cnidarian host.
Anthozoans remain polypoid throughout their lives. They can reproduce asexually by budding or fragmentation, or sexually by producing gametes. Both gametes are produced by the polyp, which can fuse to give rise to a free-swimming planula larva. The larva settles on a suitable substratum and develops into a sessile polyp. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/32%3A_Animal_Diversity_and_the_Evolution_of_Body_Plans/32.05%3A_Eumetazoa-_Animals_with_True_Tissues/32.5B%3A_Class_Anthozoa.txt |
Scyphozoans are free-swimming, polymorphic, dioecious, and carnivorous cnidarians with a prominent medusa morphology.
Learning Objectives
• Explain the key features of scyphozoa
Key Points
• Scyphozoans have a ring of muscles that lines the dome of their bodies; these structures provide them with the contractile force they need to swim through water.
• Scyphozoans have separate sexes and form planula larvae through external fertilization.
• Jellies exhibit the polyp form, known as a scyphistoma, after their larvae settle on a substrate; these forms will later bud-off and transform into their more prominenent medusa forms.
Key Terms
• dioecious: having the male and female reproductive organs on separate parts (of the same species)
• rhopalia: small sensory structures found within Scyphozoa that are characterized by clusters of neurons that can be used to sense light
• scyphistoma: the polypoid form of scyphozoans
• nematocyst: a capsule, in certain cnidarians, containing a barbed, threadlike tube that delivers a paralyzing sting
Class Scyphozoa
Class Scyphozoa, an exclusively marine class of animals with about 200 known species, includes all the jellies. The defining characteristic of this class is that the medusa is the prominent stage in the life cycle, although there is a polyp stage present. Members of this species range from 2 to 40 cm in length, but the largest scyphozoan species, Cyanea capillata, can reach a size of 2 m across. Scyphozoans display a characteristic bell-like morphology.
In the jellyfish, a mouth opening, surrounded by tentacles bearing nematocysts, is present on the underside of the animal. Scyphozoans live most of their life cycle as free-swimming, solitary carnivores. The mouth leads to the gastrovascular cavity, which may be sectioned into four interconnected sacs, called diverticuli. In some species, the digestive system may be further branched into radial canals. Like the septa in anthozoans, the branched gastrovascular cells serves to increase the surface area for nutrient absorption and diffusion; thus, more cells are in direct contact with the nutrients in the gastrovascular cavity.
In scyphozoans, nerve cells are scattered over the entire body. Neurons may even be present in clusters called rhopalia. These animals possess a ring of muscles lining the dome of the body, which provides the contractile force required to swim through water. Scyphozoans are dioecious animals, having separate sexes. The gonads are formed from the gastrodermis with gametes expelled through the mouth. Planula larvae are formed by external fertilization; they settle on a substratum in a polypoid form known as scyphistoma. These forms may produce additional polyps by budding or may transform into the medusoid form. The life cycle of these animals can be described as polymorphic because they exhibit both a medusal and polypoid body plan at some point.
32.5D: Class Cubozoa and Class Hydrozoa
Cubozoans live as box-shaped medusae while Hydrozoans are true polymorphs and can be found as colonial or solitary organisms.
Learning Objectives
• Distinguish between cubozoa and hydrozoa cnidarians
Key Points
• Cubozoans differ from Scyphozoans in their arrangement of tentacles; they are also known for their box-shaped medusa.
• Out of all cnidarians, cubozoans are the most venomous.
• Hydrozoans are polymorphs, existing as solitary polyps, solitary medusae, or as colonies.
• Hydrozoans are unique from all other cnidarians in that their gonads are derived from epidermal tissue.
Key Terms
• hydroid: any of many colonial coelenterates that exist mainly as a polyp; a hydrozoan
Class Cubozoa
Class Cubozoa includes jellies that have a box-shaped medusa: a bell that is square in cross-section; hence, they are colloquially known as “box jellyfish.” These species may achieve sizes of 15–25 cm. Cubozoans display overall morphological and anatomical characteristics that are similar to those of the scyphozoans. A prominent difference between the two classes is the arrangement of tentacles. This is the most venomous group of all the cnidarians.
The cubozoans contain muscular pads called pedalia at the corners of the square bell canopy, with one or more tentacles attached to each pedalium. These animals are further classified into orders based on the presence of single or multiple tentacles per pedalium. In some cases, the digestive system may extend into the pedalia. Nematocysts may be arranged in a spiral configuration along the tentacles; this arrangement helps to effectively subdue and capture prey. Cubozoans exist in a polypoid form that develops from a planula larva. These polyps show limited mobility along the substratum. As with scyphozoans, they may bud to form more polyps to colonize a habitat. Polyp forms then transform into the medusoid forms.
Class Hydrozoa
Hydrozoa includes nearly 3,200 species; most are marine, although some freshwater species are known. Animals in this class are polymorphs: most exhibit both polypoid and medusoid forms in their lifecycle, although this is variable.
The polyp form in these animals often shows a cylindrical morphology with a central gastrovascular cavity lined by the gastrodermis. The gastrodermis and epidermis have a simple layer of mesoglea sandwiched between them. A mouth opening, surrounded by tentacles, is present at the oral end of the animal. Many hydrozoans form colonies that are composed of a branched colony of specialized polyps that share a gastrovascular cavity, such as in the colonial hydroid Obelia. Colonies may also be free-floating and contain medusoid and polypoid individuals in the colony as in Physalia (the Portuguese Man O’ War) or Velella (By-the-wind sailor). Other species are solitary polyps (Hydra) or solitary medusae (Gonionemus). The true characteristic shared by all these diverse species is that their gonads for sexual reproduction are derived from epidermal tissue, whereas in all other cnidarians they are derived from gastrodermal tissue. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/32%3A_Animal_Diversity_and_the_Evolution_of_Body_Plans/32.05%3A_Eumetazoa-_Animals_with_True_Tissues/32.5C%3A_Class_Scyphozoa.txt |
Phylogenetic trees are constructed according to the evolutionary relationships that exist between organisms based on homologous traits.
Learning Objectives
• Describe the information needed to construct a phylogenetic tree of animals
Key Points
• Phylogenetic trees are constructed using various data derived from studies on homologous traits, analagous traits, and molecular evidence that can be used to establish relationships using polymeric molecules ( DNA, RNA, and proteins ).
• Evolutionary relationships between animal phyla, or Metazoa, are based on the the presence or absence of differentiated tissues, referred to as Eumetazoa or Parazoa, respectively.
• Eumetazoa can be further classified into categories that are based on whether they have radial or bilateral symmetry, referred to as Radiata or Bilateria, respectively.
Key Terms
• orthologous: having been separated by a speciation event
• homoplasy: a correspondence between the parts or organs of different species acquired as the result of parallel evolution or convergence
Constructing an Animal Phylogenetic Tree
Evolutionary trees, or phylogeny, is the formal study of organisms and their evolutionary history with respect to each other. Phylogenetic trees are most-commonly used to depict the relationships that exist between species. In particular, they clarify whether certain traits are homologous (found in the common ancestor as a result of divergent evolution) or homoplasy (sometimes referred to as analogous: a character that is not found in a common ancestor, but whose function developed independently in two or more organisms through convergent evolution). Evolutionary trees are diagrams that show various biological species and their evolutionary relationships. They consist of branches that flow from lower forms of life to the higher forms of life.
Evolutionary trees differ from taxonomy which is an ordered division of organisms into categories based on a set of characteristics used to assess similarities and differences. Evolutionary trees involve biological classification and use morphology to show relationships. Phylogeny is evolutionary history shown by the relationships found when comparing polymeric molecules such as RNA, DNA, or proteins of various organisms. The evolutionary pathway is analyzed by the sequence similarity of these polymeric molecules. This is based on the assumption that the similarities of sequence result from having fewer evolutionary divergences than others. The evolutionary tree is constructed by aligning the sequences; the length of the branch is proportional to the amount of amino acid differences between the sequences.
Phylogenetic systematics informs the construction of phylogenetic trees based on shared characters. Comparing nucleic acids or other molecules to infer relationships is a valuable tool for tracing an organism’s evolutionary history. The ability of molecular trees to encompass both short and long periods of time is hinged on the ability of genes to evolve at different rates, even in the same evolutionary lineage. For example, the DNA that codes for rRNA changes relatively slowly, so comparisons of DNA sequences in these genes are useful for investigating relationships between taxa that diverged a long time ago. Interestingly, 99% of the genes in humans and mice are detectably orthologous, and 50% of our genes are orthologous with those of yeast. The hemoglobin B genes in humans and in mice are orthologous. These genes serve similar functions, but their sequences have diverged since the time that humans and mice had a common ancestor.
Evolutionary pathways relating the members of a family of proteins may be deduced by examination of sequence similarity. This approach is based on the notion that sequences that are more similar to one another have had less evolutionary time to diverge than have sequences that are less similar. Evolutionary trees are used today for DNA hybridization, which determines the percentage difference of genetic material between two similar species. If there is a high resemblance of DNA between the two species, then the species are closely related. If only a small percentage is identical, then they are distantly related.
Animal Phyla
The current understanding of evolutionary relationships between animal, or Metazoa, phyla begins with the distinction between “true” animals with true differentiated tissues, called Eumetazoa, and animal phyla that do not have true differentiated tissues (such as the sponges), called Parazoa. Both Parazoa and Eumetazoa evolved from a common ancestral organism that resembles the modern-day protists called choanoflagellates. These protist cells strongly resemble sponge choanocyte cells.
Eumetazoa are subdivided into radially-symmetrical animals and bilaterally-symmetrical animals and are classified into clade Radiata or Bilateria, respectively. The cnidarians and ctenophores are animal phyla with true radial symmetry. All other Eumetazoa are members of the Bilateria clade. The bilaterally-symmetrical animals are further divided into deuterostomes (including chordates and echinoderms) and two distinct clades of protostomes (including ecdysozoans and lophotrochozoans). Ecdysozoa includes nematodes and arthropods; named for a commonly-found characteristic among the group: exoskeletal molting (termed ecdysis). Lophotrochozoa is named for two structural features, each common to certain phyla within the clade. Some lophotrochozoan phyla are characterized by a larval stage called trochophore larvae, and other phyla are characterized by the presence of a feeding structure called a lophophore.
32.06: The Bilateria
The process of establishing relationships between organisms is increasingly becoming more accurate due to advances in molecular analysis.
Learning Objectives
• Distinguish between morphological and molecular data in creating phylogenetic trees of animals
Key Points
• The construction of phylogenetic trees is now based on similarities and differences within the molecular sources used for analysis which include DNA, RNA, and proteins.
• The ability to use molecular sources as a basis of phylogenetic tree construction has allowed for determination of previously-unknown evolutionary relationships between organisms.
• In addition to the establishment of new relationships within phylogenetic trees, the ability to use molecular sources for analysis has also created an emergence of new phlyums that were previously classified in larger groups.
• Besides identifying molecular similarities and differences between organisms, by assigning a constant mutation rate to a sequence and performing a sequence alignment, it is possible to determine when two organisms diverged from one another.
Key Terms
• monophyletic: of, pertaining to, or affecting a single phylum (or other taxon) of organisms
Modern Advances in Phylogenetic Understanding Come from Molecular Analyses
The phylogenetic groupings are continually being debated and refined by evolutionary biologists. Each year, new evidence emerges that further alters the relationships described by a phylogenetic tree diagram. Previously, phylogenetic trees were constructed based on homologous and analogous morphology; however, with the advances in molecular biology, construction of phylogenetic trees is increasingly performed using data derived from molecular analyses.
Many evolutionary relationships in the modern tree have only recently been determined due to molecular evidence. Nucleic acid and protein analyses have informed the construction of the modern phylogenetic animal tree. These data come from a variety of molecular sources, such as mitochondrial DNA, nuclear DNA, ribosomal RNA (rRNA), and certain cellular proteins. Evolutionary trees can be made by the determination of sequence information of similar genes in different organisms. Sequences that are similar to each other frequently are considered to have less time to diverge, while less similar sequences have more evolutionary time to diverge. The evolutionary tree is created by aligning sequences and having each branch length proportional to the amino acid differences of the sequences. Furthermore, by assigning a constant mutation rate to a sequence and performing a sequence alignment, it is possible to calculate the approximate time when the sequence of interest diverged into monophyletic groups.
Sequence alignments can be performed on a variety of sequences. For constructing an evolutionary tree from proteins, for example, the sequences are aligned and then compared. rRNA (ribosomal RNA) is typically used to compare organisms since rRNA has a slower mutation rate and is a better source for evolutionary tree construction. This is best supported by research of Dr. Carl Woese that was conducted in the late 1970s. Since the ribosomes are critical to the function of living organisms, they are not easily changed through the process of evolution. Taking advantage of this fact, Dr. Woese compared the minuscule differences in the sequences of ribosomes among a great array of bacteria and showed that they were not all related.
For example, a previously-classified group of animals called lophophorates, which included brachiopods and bryozoans, were long-thought to be primitive deuterostomes. Extensive molecular analysis using rRNA data found these animals to be protostomes, more closely related to annelids and mollusks. This discovery allowed for the distinction of the protostome clade: the lophotrochozoans. Molecular data have also shed light on some differences within the lophotrochozoan group. Some scientists believe that the phyla Platyhelminthes and Rotifera within this group should actually belong to their own group of protostomes termed Platyzoa.
Molecular research similar to the discoveries that brought about the distinction of the lophotrochozoan clade has also revealed a dramatic rearrangement of the relationships between mollusks, annelids, arthropods, and nematodes; a new ecdysozoan clade was formed. Due to morphological similarities in their segmented body types, annelids and arthropods were once thought to be closely related. However, molecular evidence has revealed that arthropods are actually more closely related to nematodes, now comprising the ecdysozoan clade, and annelids are more closely related to mollusks, brachiopods, and other phyla in the lophotrochozoan clade. These two clades now make up the protostomes.
Another change to former phylogenetic groupings because of molecular analyses includes the emergence of an entirely new phylum of worm called Acoelomorpha. These acoel flatworms were long thought to belong to the phylum Platyhelminthes because of their similar “flatworm” morphology. However, molecular analyses revealed this to be a false relationship and originally suggested that acoels represented living species of some of the earliest divergent bilaterians. More recent research into the acoelomorphs has called this hypothesis into question and suggested a closer relationship with deuterostomes. The placement of this new phylum remains disputed, but scientists agree that with sufficient molecular data, their true phylogeny will be determined.
Contributions and Attributions
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44658/latest...ol11448/latest. License: CC BY: Attribution
• Structural Biochemistry/Bioinformatics/Evolution Trees. Provided by: Wikibooks. Located at: en.wikibooks.org/wiki/Structu...volution_Trees. License: CC BY-SA: Attribution-ShareAlike
• orthologous. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/orthologous. License: CC BY-SA: Attribution-ShareAlike
• homoplasy. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/homoplasy. License: CC BY-SA: Attribution-ShareAlike
• Tree of life SVG. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...f_life_SVG.svg. License: Public Domain: No Known Copyright
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44658/latest...ol11448/latest. License: CC BY: Attribution
• Structural Biochemistry/Bioinformatics/Evolution Trees. Provided by: Wikibooks. Located at: en.wikibooks.org/wiki/Structu...volution_Trees. License: CC BY-SA: Attribution-ShareAlike
• monophyletic. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/monophyletic. License: CC BY-SA: Attribution-ShareAlike
• Tree of life SVG. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...f_life_SVG.svg. License: Public Domain: No Known Copyright
• PhylogeneticTree. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...eneticTree.png. License: Public Domain: No Known Copyright | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/32%3A_Animal_Diversity_and_the_Evolution_of_Body_Plans/32.06%3A_The_Bilateria/32.6B%3A_Molecular_Analyses_and_Modern_Phylogenetic_Trees.txt |
The Lophotrochozoa are protostomes possessing a blastopore, an early form of a mouth; they include the trochozoans and the lophophorata.
Learning Objectives
• Describe the phylogenetic position and basic features of lophotrochozoa
Key Points
• Lophotrochozoa have a blastopore, which is an involution of the ectoderm that forms a rudimentary mouth opening to the alimentary canal, a condition called protostomy or “first mouth”.
• The Lophotrochozoa are comprised of the trochozoans and the lophophorata, although the exact relationships between the different phyla are not clearly determined.
• Lophophores are characterized by the presence of the lophophore, a set of ciliated tentacles surrounding the mouth; they include the flatworms and several other phyla whose relationships are upheld by genetic evidence.
• Trochophore larvae are distinguished from the lophophores by two bands of cilia around the body; they include the Nemertea, Mollusca, Sipuncula, and Annelida.
• The lophotrochozoans have a mesoderm layer positioned between the ectoderm and endoderm and are bilaterally symmetrical, which signals the beginning of cephalization, the concentration of nervous tissues and sensory organs in the head of the organism.
Key Terms
• blastopore: the opening into the archenteron
• lophophore: a feeding organ of brachiopods, bryozoans, and phoronids
• cephalization: an evolutionary trend in which the neural and sense organs become centralized at one end (the head) of an animal
Lophotrochozoans
Animals belonging to superphylum Lophotrochozoa are protostomes: the blastopore (or the point of involution of the ectoderm or outer germ layer) becomes the mouth opening to the alimentary canal. This is called protostomy or “first mouth.” In protostomy, solid groups of cells split from the endoderm or inner germ layer to form a central mesodermal layer of cells. This layer multiplies into a band which then splits internally to form the coelom; this protostomic coelom is termed schizocoelom.
As lophotrochozoans, the organisms in this superphylum possess either lophophore or trochophore larvae. The exact relationships between the different phyla are not entirely certain. The lophophores include groups that are united by the presence of the lophophore, a set of ciliated tentacles surrounding the mouth. Lophophorata include the flatworms and several other phyla, including the Bryozoa, Entoprocta, Phoronida, and Brachiopoda. These clades are upheld when RNA sequences are compared. Trochophore larvae are characterized by two bands of cilia around the body. Previously, these were treated together as the Trochozoa, together with the arthropods, which do not produce trochophore larvae, but were considered close relatives of the annelids because they are both segmented. However, they show a number of important differences. Arthropods are now placed separately among the Ecdysozoa. The Trochozoa include the Nemertea, Mollusca, Sipuncula, and Annelida.
The lophotrochozoans are triploblastic, possessing an embryonic mesoderm sandwiched between the ectoderm and endoderm found in the diploblastic cnidarians. These phyla are also bilaterally symmetrical: a longitudinal section will divide them into right and left sides that are symmetrical. They also show the beginning of cephalization: the evolution of a concentration of nervous tissues and sensory organs in the head of the organism, which is where it first encounters its environment.
33.1B: Phylum Platyhelminthes
The Platyhelminthes are flatworms that lack a coelom; many are parasitic; all lack either a circulatory or respiratory system.
Learning Objectives
• Differentiate among the classes of platyhelminthes
Key Points
• The Platyhelminthes are acoelomate flatworms: their bodies are solid between the outer surface and the cavity of the digestive system.
• Most flatworms have a gastrovascular cavity rather than a complete digestive system; the same cavity used to bring in food is used to expel waste materials.
• Platyhelminthes are either predators or scavengers; many are parasites that feed on the tissues of their hosts.
• Flatworms posses a simple nervous system, no circulatory or respiratory system, and most produce both eggs and sperm, with internal fertilization.
• Platyhelminthes are divided into four classes: Turbellaria, free-living marine species; Monogenea, ectoparasites of fish; Trematoda, internal parasites of humans and other species; and Cestoda (tapeworms), which are internal parasites of many vertebrates.
• In flatworms, digested materials are taken into the cells of the gut lining by phagocytosis, rather than being processed internally.
Key Terms
• acoelomate: any animal without a coelom, or body cavity
• ectoparasite: a parasite that lives on the surface of a host organism
• scolex: the structure at the rear end of a tapeworm which, in the adult, has suckers and hooks by which it attaches itself to a host
• proglottid: any of the segments of a tapeworm; they contain both male and female reproductive organs
Phylum Platyhelminthes
Phylum Platyhelminthes is composed of the flatworms: acoelomate organisms that include many free-living and parasitic forms. Most of the flatworms are classified in the superphylum Lophotrochozoa, which also includes the mollusks and annelids. The Platyhelminthes consist of two lineages: the Catenulida and the Rhabditophora. The Catenulida, or “chain worms” is a small clade of just over 100 species. These worms typically reproduce asexually by budding. However, the offspring do not fully detach from the parents; therefore, they resemble a chain. The remaining flatworms discussed here are part of the Rhabditophora.
Many flatworms are parasitic, including important parasites of humans. Flatworms have three embryonic tissue layers that give rise to surfaces that cover tissues (from ectoderm), internal tissues (from mesoderm), and line the digestive system (from endoderm). The epidermal tissue is a single layer cells or a layer of fused cells (syncytium) that covers a layer of circular muscle above a layer of longitudinal muscle. The mesodermal tissues include mesenchymal cells that contain collagen and support secretory cells that secrete mucus and other materials at the surface. The flatworms are acoelomates: their bodies are solid between the outer surface and the cavity of the digestive system.
Physiological Processes of Flatworms
The free-living species of flatworms are predators or scavengers. Parasitic forms feed on the tissues of their hosts. Most flatworms have a gastrovascular cavity rather than a complete digestive system; in such animals, the “mouth” is also used to expel waste materials from the digestive system. Some species also have an anal opening. The gut may be a simple sac or highly branched. Digestion is extracellular, with digested materials taken in to the cells of the gut lining by phagocytosis. One group, the cestodes, lacks a digestive system. Flatworms have an excretory system with a network of tubules throughout the body with openings to the environment and nearby flame cells, whose cilia beat to direct waste fluids concentrated in the tubules out of the body. The system is responsible for the regulation of dissolved salts and the excretion of nitrogenous wastes. The nervous system consists of a pair of nerve cords running the length of the body with connections between them and a large ganglion or concentration of nerves at the anterior end of the worm, where there may also be a concentration of photosensory and chemosensory cells.
There is neither a circulatory nor respiratory system, with gas and nutrient exchange dependent on diffusion and cell-cell junctions. This necessarily limits the thickness of the body in these organisms, constraining them to be “flat” worms. In addition, most flatworm species are monoecious; typically, fertilization is internal. Asexual reproduction is common in some groups.
Diversity of Flatworms
Platyhelminthes are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda. The class Turbellaria includes mainly free-living, marine species, although some species live in freshwater or moist terrestrial environments. The ventral epidermis of turbellarians is ciliated which facilitates their locomotion. Some turbellarians are capable of remarkable feats of regeneration: they may regrow the entire body from a small fragment.
The monogeneans are ectoparasites, mostly of fish, with simple life cycles that consist of a free-swimming larva that attaches to a fish to begin transformation to the parasitic adult form. The worms may produce enzymes that digest the host tissues or simply graze on surface mucus and skin particles.
The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. Trematodes have complex life cycles that involve a primary host in which sexual reproduction occurs and one or more secondary hosts in which asexual reproduction occurs. The primary host is almost always a mollusk. Trematodes are responsible for serious human diseases including schistosomiasis, a blood fluke.
The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates. Tapeworms live in the intestinal tract of the primary host, remaining fixed by using a sucker on the anterior end, or scolex, of the tapeworm body. The remainder of the tapeworm is composed of a long series of units called proglottids. Each may contain an excretory system with flame cells and both female and male reproductive structures. Tapeworms do not possess a digestive system; instead, they absorb nutrients from the food matter passing through them in the host’s intestine. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.01%3A_The_Clades_of_Protostomes/33.1A%3A_Superphylum_Lophotrochozoa.txt |
Rotifers are microscopic organisms named for a rotating structure (called the corona) at their anterior end that is covered with cilia.
Learning Objectives
• Identify the features of rotifers involved in movement and feeding
Key Points
• The rotifer body form consists of a head (containing the sensory organs in the form of a bi-lobed brain and small eyespots near the corona), the trunk (containing organs), and the foot (which can hold fast).
• The foot of the rotifer secretes a sticky material to help it adhere to surfaces.
• Rotifers are filter feeders that generate a current using the corona to pass food into the mouth, which then passes by digestive and salivary glands into the stomach and intestines.
• Rotifers exhibit sexual dimorphism; the gender of many species is determined by whether the egg is fertilized (and develops into a female) or unfertilized (and develops into a male).
Key Terms
• pseudocoelomate: any invertebrate animal with a three-layered body and a pseudocoel
• mastax: the pharynx of a rotifer which usually contains four horny pieces that work to crush the food
Phylum Rotifera
The rotifers are a microscopic (about 100 µm to 30 mm) group of mostly-aquatic organisms that get their name from the corona: a rotating, wheel-like structure that is covered with cilia at their anterior end. Although their taxonomy is currently in flux, one treatment places the rotifers in three classes: Bdelloidea, Monogononta, and Seisonidea. The classification of the group is currently under revision, however, as more phylogenetic evidence becomes available. It is possible that the “spiny headed worms” currently in phylum Acanthocephala will be incorporated into this group in the future.
The rotifer body form consists of a head (which contains the corona), a trunk (which contains the organs), and the foot. Rotifers are typically free-swimming and truly planktonic organisms, but the toes or extensions of the foot can secrete a sticky material forming a holdfast to help them adhere to surfaces. The head contains sensory organs in the form of a bi-lobed brain and small eyespots near the corona.
The rotifers are filter feeders that will eat dead material, algae, and other microscopic living organisms. Therefore, they are very important components of aquatic food webs. Rotifers obtain food that is directed toward the mouth by the current created from the movement of the corona. The food particles enter the mouth and travel to the mastax (pharynx with jaw-like structures). Food passes by digestive and salivary glands into the stomach and then into the intestines. Digestive and excretory wastes are collected in a cloacal bladder before being released out the anus.
Rotifers are pseudocoelomates commonly found in fresh water and some salt water environments throughout the world. About 2,200 species of rotifers have been identified. Rotifers are dioecious organisms (having either male or female genitalia) and exhibit sexual dimorphism (males and females have different forms). Many species are parthenogenic and exhibit haplodiploidy, a method of gender determination in which a fertilized egg develops into a female and an unfertilized egg develops into a male. In many dioecious species, males are short-lived and smaller, with no digestive system and a single testis. Females can produce eggs that are capable of dormancy, which protects eggs during harsh environmental conditions. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.01%3A_The_Clades_of_Protostomes/33.1C%3A_Phylum_Rotifera.txt |
Nemertea, or ribbon worms, are distinguished by their proboscis, used for capturing prey and enclosed in a cavity called a rhynchocoel.
Learning Objectives
• Identify the key features of the Phylum Nemertea
Key Points
• The Nemertini are mostly bottom-dwelling marine organisms, although some are found in freshwater and terrestrial habitats.
• Most nemerteans are carnivores, some are scavengers, and others have evolved relationships with some mollusks that are benefit the Nemertean but do not harm the mollusk.
• Nemerteans vary greatly in size and are bilaterally symmetrical; they are unsegmented and resemble a flat tube which can change morphological presentation in response to environmental cues.
• Nemertini have a simple nervous system comprised of a ring of four nerve masses called “ganglia” at the anterior end between the mouth and the foregut from which paired longitudinal nerve cords emerge and extend to the posterior end.
• Nemertini are mostly sexually dimorphic, fertilizing eggs externally by releasing both eggs and sperm into the water; a larva may develop inside the resulting young worm and devour its tissues before metamorphosing into the adult.
Key Terms
• protonephridia: an invertebrate organ which occurs in pairs and removes metabolic wastes from an animal’s body
• rhynchocoel: a cavity which mostly runs above the midline and ends a little short of the rear of the body of a nemertean and extends or retracts the proboscis
• proboscis: an elongated tube from the head or connected to the mouth, of an animal
Phylum Nemertea
The Nemertea are colloquially known as ribbon worms. Most species of phylum Nemertea are marine (predominantly benthic or bottom dwellers) with an estimated 900 species known. However, nemertini have been recorded in freshwater and terrestrial habitats as well. Most nemerteans are carnivores, feeding on worms, clams, and crustaceans. Some species are scavengers, while other nemertini species, such as Malacobdella grossa, have also evolved commensalistic relationships with some mollusks. Interestingly, nemerteans have almost no predators, two species are sold as fish bait, and some species have devastated commercial fishing of clams and crabs.
Morphology
Ribbon worms vary in size from 1 cm to several meters. They show bilateral symmetry and remarkable contractile properties. Because of their contractility, they can change their morphological presentation in response to environmental cues. Animals in phylum Nemertea also show a flattened morphology: they are flat from front to back, like a flattened tube. In addition, nemertea are soft, unsegmented animals.
A unique characteristic of this phylum is the presence of a proboscis enclosed in a rhynchocoel. The proboscis serves to capture food and may be ornamented with barbs in some species. The rhynchocoel is a fluid-filled cavity that extends from the head to nearly two-thirds of the length of the gut in these animals. The proboscis may be extended or retracted by the retractor muscle attached to the wall of the rhynchocoel.
Metabolism
The nemertini show a very well-developed digestive system. A mouth opening that is ventral to the rhynchocoel leads into the foregut, followed by the intestine. The intestine is present in the form of diverticular pouches which ends in a rectum that opens via an anus. Gonads are interspersed with the intestinal diverticular pouches, opening outwards via genital pores. A circulatory system consists of a closed loop of a pair of lateral blood vessels. The circulatory system is derived from the coelomic cavity of the embryo. Some animals may also have cross-connecting vessels in addition to lateral ones. Although these are called blood vessels, since they are of coelomic origin, the circulatory fluid is colorless. Some species bear hemoglobin as well as yellow or green pigments. The blood vessels are connected to the rhynchocoel. The flow of fluid in these vessels is facilitated by the contraction of muscles in the body wall. A pair of protonephridia, or primitive kidneys, is present in these animals to facilitate osmoregulation. Gaseous exchange occurs through the skin in the nemertini.
Nervous System
Nemertini have a ganglion or “brain” situated at the anterior end between the mouth and the foregut, surrounding the digestive system as well as the rhynchocoel. A ring of four nerve masses called “ganglia” comprises the brain in these animals. Paired longitudinal nerve cords emerge from the brain ganglia, extending to the posterior end. Ocelli or eyespots are present in pairs, in multiples of two in the anterior portion of the body. It is speculated that the eyespots originate from neural tissue and not from the epidermis.
Reproduction
Animals in phylum Nemertea show sexual dimorphism, although freshwater species may be hermaphroditic. Eggs and sperm are released into the water; fertilization occurs externally. The zygote develops into a special kind of nemertean larvae called a planuliform larva. In some nemertine species, another larva specific to the nemertinis, a pilidium, may develop inside the young worm from a series of imaginal discs. This larval form, characteristically shaped like a deerstalker cap, devours tissues from the young worm for survival before metamorphosing into the adult-like morphology. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.01%3A_The_Clades_of_Protostomes/33.1D%3A_Phylum_Nemertea.txt |
Mollusks have a soft body and share several characteristics, including a muscular foot, a visceral mass of internal organs, and a mantle.
Learning Objectives
• Describe the unique anatomical and morphological features of molluscs
Key Points
• A mollusk’s muscular foot is used for locomotion and anchorage, varies in shape and function, and can both extend and retract.
• The visceral mass inside the mollusk includes digestive, nervous, excretory, reproductive, and respiratory systems.
• Most mollusks possess a radula, which is similar to a tongue with teeth-like projections, serving to shred or scrape food.
• The mantle is the dorsal epidermis in mollusks; in some mollusks it secretes a chitinous and hard calcareous shell.
Key Terms
• visceral mass: the soft, non-muscular metabolic region of the mollusc that contains the body organs
• mantle: the body wall of a mollusc, from which the shell is secreted
• radula: the rasping tongue of snails and most other mollusks
Phylum Mollusca
Phylum Mollusca is the predominant phylum in marine environments. It is estimated that 23 percent of all known marine species are mollusks; there are around 85,000 described species, making them the second most diverse phylum of animals. The name “mollusca” signifies a soft body; the earliest descriptions of mollusks came from observations of unshelled cuttlefish. Mollusks are predominantly a marine group of animals; however, they are known to inhabit freshwater as well as terrestrial habitats. Mollusks display a wide range of morphologies in each class and subclass. They range from large predatory squids and octopus, some of which show a high degree of intelligence, to grazing forms with elaborately-sculpted and colored shells. In spite of their tremendous diversity, however, they also share a few key characteristics, including a muscular foot, a visceral mass containing internal organs, and a mantle that may or may not secrete a shell of calcium carbonate.
Mollusks have a muscular foot used for locomotion and anchorage that varies in shape and function, depending on the type of mollusk under study. In shelled mollusks, this foot is usually the same size as the opening of the shell. The foot is a retractable as well as an extendable organ. It is the ventral-most organ, whereas the mantle is the limiting dorsal organ. Mollusks are eucoelomate, but the cavity is restricted to a region around the heart in adult animals. The mantle cavity develops independently of the coelomic cavity.
The visceral mass is present above the foot in the visceral hump. This includes digestive, nervous, excretory, reproductive, and respiratory systems. Mollusk species that are exclusively aquatic have gills for respiration, whereas some terrestrial species have lungs for respiration. Additionally, a tongue-like organ called a radula, which bears chitinous tooth-like ornamentation, is present in many species, serving to shred or scrape food. The mantle (also known as the pallium) is the dorsal epidermis in mollusks; shelled mollusks are specialized to secrete a chitinous and hard calcareous shell.
Most mollusks are dioecious animals where fertilization occurs externally, although this is not the case in terrestrial mollusks, such as snails and slugs, or in cephalopods. In some mollusks, the zygote hatches and undergoes two larval stages, trochophore and veliger, before becoming a young adult; bivalves may exhibit a third larval stage, glochidia. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.01%3A_The_Clades_of_Protostomes/33.1E%3A_Phylum_Mollusca.txt |
The phylum Mollusca includes a wide variety of animals including the gastropods (“stomach foot”), the cephalopods (“head foot”), and the scaphopods (“boat foot”).
Learning Objectives
• Differentiate among the classes in the phylum mollusca
Key Points
• Mollusks can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. These classes are distinguished by, among other criteria, the presence and types of shells they possess.
• Class Aplacophora includes worm-like animals with no shell and a rudimentary body structure.
• Members of class Monoplacophora have a single shell that encloses the body.
• Members of class Polyplacophora are better known as “chitons;” these molluscs have a large foot on the ventral side and a shell composed of eight hard plates on the dorsal side.
• Class Bivalvia consists of mollusks with two shells held together by a muscle; these include oysters, clams, and mussels.
• Members of class Gastropoda have an asymmetrical body plan and usually have a shell, which can be planospiral or conispiral. Their key characteristic is the torsion around the perpendicular axis on the center of the foot that is modified for crawling.
• Class Scaphopoda consists of mollusks with a single conical shell through which the head protrudes, and a foot modified into tentacles known as captaculae that are used to catch and manipulate prey.
Key Terms
• ctenidium: a respiratory system, in the form of a comb, in some molluscs
• captacula: the foot of a Scaphalopod, modified into tentacles for capturing prey
• nephridium: a tubular excretory organ in some invertebrates
Classes in Phylum Mollusca
Phylum Mollusca is a very diverse (85,000 species ) group of mostly marine species, with a dramatic variety of form. This phylum can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda.
Class Aplacophora
Class Aplacophora (“bearing no plates”) includes worm-like animals primarily found in benthic marine habitats. These animals lack a calcareous shell, but possess aragonite spicules on their epidermis. They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia (excretory organs).
Class Monoplacophora
Members of class Monoplacophora (“bearing one plate”) posses a single, cap-like shell that encloses the body. The morphology of the shell and the underlying animal can vary from circular to ovate. A looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads are present in these animals. The monoplacophorans were believed extinct and only known via fossil records until the discovery of Neopilina galathaea in 1952. Today, scientists have identified nearly two dozen extant species.
Class Polyplacophora
Animals in the class Polyplacophora (“bearing many plates”) are commonly known as “chitons” and bear an armor-like, eight-plated dorsal shell. These animals have a broad, ventral foot that is adapted for suction to rocks and other substrates, and a mantle that extends beyond the shell in the form of a girdle. Calcareous spines may be present on the girdle to offer protection from predators. Chitons live worldwide, in cold water, warm water, and the tropics. Most chiton species inhabit intertidal or subtidal zones, and do not extend beyond the photic zone. Some species live quite high in the intertidal zone and are exposed to the air and light for long periods.
Class Bivalvia
Bivalvia is a class of marine and freshwater molluscs with laterally compressed bodies enclosed by a shell in two hinged parts. Bivalves include clams, oysters, mussels, scallops, and numerous other families of shells. The majority are filter feeders and have no head or radula. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment on the seabed, while others lie on the sea floor or attach themselves to rocks or other hard surfaces.
The shell of a bivalve is composed of calcium carbonate, and consists of two, usually similar, parts called valves. These are joined together along one edge by a flexible ligament that, in conjunction with interlocking “teeth” on each of the valves, forms the hinge.
Class Gastropoda
Animals in class Gastropoda (“stomach foot”) include well-known mollusks like snails, slugs, conchs, sea hares, and sea butterflies. Gastropoda includes shell-bearing species as well as species with a reduced shell. These animals are asymmetrical and usually present a coiled shell. Shells may be planospiral (like a garden hose wound up), commonly seen in garden snails, or conispiral (like a spiral staircase), commonly seen in marine conches.
The visceral mass in the shelled species displays torsion around the perpendicular axis on the center of the foot, which is the key characteristic of this group, along with a foot that is modified for crawling. Most gastropods bear a head with tentacles, eyes, and a style. A complex radula is used by the digestive system and aids in the ingestion of food. Eyes may be absent in some gastropods species. The mantle cavity encloses the ctenidia (singluar: ctenidium) as well as a pair of nephridia (singular: nephridium).
Class Cephalopoda
Class Cephalopoda (“head foot” animals) includes octopuses, squids, cuttlefish, and nautilus. Cephalopods are a class of shell-bearing animals as well as mollusks with a reduced shell. They display vivid coloration, typically seen in squids and octopuses which is used for camouflage. All animals in this class are carnivorous predators and have beak-like jaws at the anterior end. All cephalopods show the presence of a very well-developed nervous system along with eyes, as well as a closed circulatory system. The foot is lobed and developed into tentacles and a funnel, which is used as the mode of locomotion. Locomotion in cephalopods is facilitated by ejecting a stream of water for propulsion (“jet” propulsion). Cephalopods, such as squids and octopuses, also produce sepia or a dark ink, which is squirted upon a predator to assist in a quick getaway. Suckers are present on the tentacles in octopuses and squid. Ctenidia are enclosed in a large mantle cavity serviced by blood vessels, each with its own associated heart. The mantle has siphonophores that facilitate exchange of water.
A pair of nephridia is present within the mantle cavity. Sexual dimorphism is seen in this class of animals. Members of a species mate, then the female lays the eggs in a secluded and protected niche. Females of some species care for the eggs for an extended period of time and may end up dying during that time period. Reproduction in cephalopods is different from other mollusks in that the egg hatches to produce a juvenile adult without undergoing the trochophore and veliger larval stages.
Class Scaphopoda
Members of class Scaphopoda (“boat feet”) are known colloquially as “tusk shells” or “tooth shells,” as evident when examining Dentalium, one of the few remaining scaphopod genera. Scaphopods are usually buried in sand with the anterior opening exposed to water. These animals bear a single conical shell, which has both ends open. The head is rudimentary and protrudes out of the posterior end of the shell. These animals do not possess eyes, but they have a radula, as well as a foot modified into tentacles with a bulbous end, known as captaculae. Captaculae serve to catch and manipulate prey. Ctenidia are absent in these animals. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.01%3A_The_Clades_of_Protostomes/33.1F%3A_Classification_of_Phylum_Mollusca.txt |
Annelids include segmented worms, such as leeches and earthworms; they are the most advanced worms as they possess a true coelom.
Learning Objectives
• Describe the morphological and anatomical features of annelids
Key Points
• Annelids are often called “segmented worms” because they possess true segmentation of their bodies, with both internal and external morphological features repeated in each body segment.
• The clitellum is a structure on the anterior portion of the worm that generates mucus to aid in sperm transfer from one worm to another; it also forms a cocoon within which fertilization occurs.
• Most annelids have chitinous hairlike extensions in every segment called chaetae that are anchored in the epidermis, although the number and size of chaetae can vary in the different classes.
• Annelids possess a closed circulatory system, lack a well-developed respiratory system, but have well-developed nervous systems.
• Annelids can either have distinct male and female forms or be hermaphrodites (having both male and female reproductive organs). Earthworms are hermaphrodites and can self-fertilize, but prefer to cross-fertilize if possible.
Key Terms
• clitellum: a glandular swelling in the epidermis of some annelid worms; it secretes a viscous fluid in which the eggs are deposited
• chaeta: a chitinous bristle of an annelid worm
• metamerism: the segmentation of the body into similar discrete units
Phylum Annelida
Phylum Annelida contains the class Polychaeta (the polychaetes) and the class Oligochaeta (the earthworms, leeches, and their relatives). These animals are found in marine, terrestrial, and freshwater habitats, but a presence of water or humidity is a critical factor for their survival, especially in terrestrial habitats. The name of the phylum is derived from the Latin word annellus, which means a small ring. Animals in this phylum show parasitic and commensal symbioses with other species in their habitat. Approximately 16,500 species have been described in phylum Annelida. The phylum includes earthworms, polychaete worms, and leeches. Annelids show protostomic development in embryonic stages and are often called “segmented worms” due to their key characteristic of metamerism, or true segmentation.
Morphology
Annelids display bilateral symmetry and are worm-like in overall morphology. They have a segmented body plan where the internal and external morphological features are repeated in each body segment. Metamerism allows animals to become bigger by adding “compartments,” while making their movement more efficient. This metamerism is thought to arise from identical teloblast cells in the embryonic stage, which develop into identical mesodermal structures. The overall body can be divided into head, body, and pygidium (or tail). The clitellum is a reproductive structure that generates mucus that aids in sperm transfer and gives rise to a cocoon within which fertilization occurs; it appears as a fused band in the anterior third of the animal.
Anatomy
The epidermis is protected by an acellular, external cuticle, but this is much thinner than the cuticle found in the ecdysozoans and does not require periodic shedding for growth. Circular as well as longitudinal muscles are located interior to the epidermis. Chitinous hairlike extensions, anchored in the epidermis and projecting from the cuticle, called setae/chaetae are present in every segment. Annelids show the presence of a true coelom, derived from embryonic mesoderm and protostomy. Hence, they are the most advanced worms. A well-developed and complete digestive system is present in earthworms (oligochaetes) with a mouth, muscular pharynx, esophagus, crop, and gizzard being present. The gizzard leads to the intestine and ends in an anal opening. Each segment is limited by a membranous septum that divides the coelomic cavity into a series of compartments.
Annelids possess a closed circulatory system of dorsal and ventral blood vessels that run parallel to the alimentary canal as well as capillaries that service individual tissues. In addition, these vessels are connected by transverse loops in every segment. These animals lack a well-developed respiratory system; gas exchange occurs across the moist body surface. Excretion is facilitated by a pair of metanephridia (a type of primitive “kidney” that consists of a convoluted tubule and an open, ciliated funnel) that is present in every segment towards the ventral side. Annelids show well-developed nervous systems with a nerve ring of fused ganglia present around the pharynx. The nerve cord is ventral in position, bearing enlarged nodes or ganglia in each segment.
Annelids may be either monoecious, with permanent gonads (as in earthworms and leeches), or dioecious, with temporary or seasonal gonads that develop (as in polychaetes). However, cross-fertilization is preferred in hermaphroditic animals. These animals may also show simultaneous hermaphroditism, participating in simultaneous sperm exchange when they are aligned for copulation.
Earthworms are the most abundant members of the class Oligochaeta, distinguished by the presence of the clitellum as well as few, reduced chaetae (“oligo- = “few”; -chaetae = “hairs”). The number and size of chaetae are greatly diminished in Oligochaeta compared to the polychaetes (poly=many, chaetae = hairs). The many chetae of polychaetes are also arranged within fleshy, flat, paired appendages that protrude from each segment. These parapodia may be specialized for different functions in the polychates. A significant difference between leeches and other annelids is the development of suckers at the anterior and posterior ends and an absence of chaetae. Additionally, the segmentation of the body wall may not correspond to the internal segmentation of the coelomic cavity. This adaptation possibly helps the leeches to elongate when they ingest copious quantities of blood from host vertebrates.
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• Egel als Schneckenparasit 04. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Egel_als_Schneckenparasit_04.JPG. License: Public Domain: No Known Copyright | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.01%3A_The_Clades_of_Protostomes/33.1G%3A_Phylum_Annelida.txt |
The Lophotrochozoa are protostomes possessing a blastopore, an early form of a mouth; they include the trochozoans and the lophophorata.
Learning Objectives
• Describe the phylogenetic position and basic features of lophotrochozoa
Key Points
• Lophotrochozoa have a blastopore, which is an involution of the ectoderm that forms a rudimentary mouth opening to the alimentary canal, a condition called protostomy or “first mouth”.
• The Lophotrochozoa are comprised of the trochozoans and the lophophorata, although the exact relationships between the different phyla are not clearly determined.
• Lophophores are characterized by the presence of the lophophore, a set of ciliated tentacles surrounding the mouth; they include the flatworms and several other phyla whose relationships are upheld by genetic evidence.
• Trochophore larvae are distinguished from the lophophores by two bands of cilia around the body; they include the Nemertea, Mollusca, Sipuncula, and Annelida.
• The lophotrochozoans have a mesoderm layer positioned between the ectoderm and endoderm and are bilaterally symmetrical, which signals the beginning of cephalization, the concentration of nervous tissues and sensory organs in the head of the organism.
Key Terms
• blastopore: the opening into the archenteron
• lophophore: a feeding organ of brachiopods, bryozoans, and phoronids
• cephalization: an evolutionary trend in which the neural and sense organs become centralized at one end (the head) of an animal
Lophotrochozoans
Animals belonging to superphylum Lophotrochozoa are protostomes: the blastopore (or the point of involution of the ectoderm or outer germ layer) becomes the mouth opening to the alimentary canal. This is called protostomy or “first mouth.” In protostomy, solid groups of cells split from the endoderm or inner germ layer to form a central mesodermal layer of cells. This layer multiplies into a band which then splits internally to form the coelom; this protostomic coelom is termed schizocoelom.
As lophotrochozoans, the organisms in this superphylum possess either lophophore or trochophore larvae. The exact relationships between the different phyla are not entirely certain. The lophophores include groups that are united by the presence of the lophophore, a set of ciliated tentacles surrounding the mouth. Lophophorata include the flatworms and several other phyla, including the Bryozoa, Entoprocta, Phoronida, and Brachiopoda. These clades are upheld when RNA sequences are compared. Trochophore larvae are characterized by two bands of cilia around the body. Previously, these were treated together as the Trochozoa, together with the arthropods, which do not produce trochophore larvae, but were considered close relatives of the annelids because they are both segmented. However, they show a number of important differences. Arthropods are now placed separately among the Ecdysozoa. The Trochozoa include the Nemertea, Mollusca, Sipuncula, and Annelida.
The lophotrochozoans are triploblastic, possessing an embryonic mesoderm sandwiched between the ectoderm and endoderm found in the diploblastic cnidarians. These phyla are also bilaterally symmetrical: a longitudinal section will divide them into right and left sides that are symmetrical. They also show the beginning of cephalization: the evolution of a concentration of nervous tissues and sensory organs in the head of the organism, which is where it first encounters its environment.
33.2B: Phylum Platyhelminthes
The Platyhelminthes are flatworms that lack a coelom; many are parasitic; all lack either a circulatory or respiratory system.
Learning Objectives
• Differentiate among the classes of platyhelminthes
Key Points
• The Platyhelminthes are acoelomate flatworms: their bodies are solid between the outer surface and the cavity of the digestive system.
• Most flatworms have a gastrovascular cavity rather than a complete digestive system; the same cavity used to bring in food is used to expel waste materials.
• Platyhelminthes are either predators or scavengers; many are parasites that feed on the tissues of their hosts.
• Flatworms posses a simple nervous system, no circulatory or respiratory system, and most produce both eggs and sperm, with internal fertilization.
• Platyhelminthes are divided into four classes: Turbellaria, free-living marine species; Monogenea, ectoparasites of fish; Trematoda, internal parasites of humans and other species; and Cestoda (tapeworms), which are internal parasites of many vertebrates.
• In flatworms, digested materials are taken into the cells of the gut lining by phagocytosis, rather than being processed internally.
Key Terms
• acoelomate: any animal without a coelom, or body cavity
• ectoparasite: a parasite that lives on the surface of a host organism
• scolex: the structure at the rear end of a tapeworm which, in the adult, has suckers and hooks by which it attaches itself to a host
• proglottid: any of the segments of a tapeworm; they contain both male and female reproductive organs
Phylum Platyhelminthes
Phylum Platyhelminthes is composed of the flatworms: acoelomate organisms that include many free-living and parasitic forms. Most of the flatworms are classified in the superphylum Lophotrochozoa, which also includes the mollusks and annelids. The Platyhelminthes consist of two lineages: the Catenulida and the Rhabditophora. The Catenulida, or “chain worms” is a small clade of just over 100 species. These worms typically reproduce asexually by budding. However, the offspring do not fully detach from the parents; therefore, they resemble a chain. The remaining flatworms discussed here are part of the Rhabditophora.
Many flatworms are parasitic, including important parasites of humans. Flatworms have three embryonic tissue layers that give rise to surfaces that cover tissues (from ectoderm), internal tissues (from mesoderm), and line the digestive system (from endoderm). The epidermal tissue is a single layer cells or a layer of fused cells (syncytium) that covers a layer of circular muscle above a layer of longitudinal muscle. The mesodermal tissues include mesenchymal cells that contain collagen and support secretory cells that secrete mucus and other materials at the surface. The flatworms are acoelomates: their bodies are solid between the outer surface and the cavity of the digestive system.
Physiological Processes of Flatworms
The free-living species of flatworms are predators or scavengers. Parasitic forms feed on the tissues of their hosts. Most flatworms have a gastrovascular cavity rather than a complete digestive system; in such animals, the “mouth” is also used to expel waste materials from the digestive system. Some species also have an anal opening. The gut may be a simple sac or highly branched. Digestion is extracellular, with digested materials taken in to the cells of the gut lining by phagocytosis. One group, the cestodes, lacks a digestive system. Flatworms have an excretory system with a network of tubules throughout the body with openings to the environment and nearby flame cells, whose cilia beat to direct waste fluids concentrated in the tubules out of the body. The system is responsible for the regulation of dissolved salts and the excretion of nitrogenous wastes. The nervous system consists of a pair of nerve cords running the length of the body with connections between them and a large ganglion or concentration of nerves at the anterior end of the worm, where there may also be a concentration of photosensory and chemosensory cells.
There is neither a circulatory nor respiratory system, with gas and nutrient exchange dependent on diffusion and cell-cell junctions. This necessarily limits the thickness of the body in these organisms, constraining them to be “flat” worms. In addition, most flatworm species are monoecious; typically, fertilization is internal. Asexual reproduction is common in some groups.
Diversity of Flatworms
Platyhelminthes are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda. The class Turbellaria includes mainly free-living, marine species, although some species live in freshwater or moist terrestrial environments. The ventral epidermis of turbellarians is ciliated which facilitates their locomotion. Some turbellarians are capable of remarkable feats of regeneration: they may regrow the entire body from a small fragment.
The monogeneans are ectoparasites, mostly of fish, with simple life cycles that consist of a free-swimming larva that attaches to a fish to begin transformation to the parasitic adult form. The worms may produce enzymes that digest the host tissues or simply graze on surface mucus and skin particles.
The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. Trematodes have complex life cycles that involve a primary host in which sexual reproduction occurs and one or more secondary hosts in which asexual reproduction occurs. The primary host is almost always a mollusk. Trematodes are responsible for serious human diseases including schistosomiasis, a blood fluke.
The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates. Tapeworms live in the intestinal tract of the primary host, remaining fixed by using a sucker on the anterior end, or scolex, of the tapeworm body. The remainder of the tapeworm is composed of a long series of units called proglottids. Each may contain an excretory system with flame cells and both female and male reproductive structures. Tapeworms do not possess a digestive system; instead, they absorb nutrients from the food matter passing through them in the host’s intestine. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.02%3A_Flatworms_%28Platyhelminthes%29/33.2A%3A_Superphylum_Lophotrochozoa.txt |
The Lophotrochozoa are protostomes possessing a blastopore, an early form of a mouth; they include the trochozoans and the lophophorata.
Learning Objectives
• Describe the phylogenetic position and basic features of lophotrochozoa
Key Points
• Lophotrochozoa have a blastopore, which is an involution of the ectoderm that forms a rudimentary mouth opening to the alimentary canal, a condition called protostomy or “first mouth”.
• The Lophotrochozoa are comprised of the trochozoans and the lophophorata, although the exact relationships between the different phyla are not clearly determined.
• Lophophores are characterized by the presence of the lophophore, a set of ciliated tentacles surrounding the mouth; they include the flatworms and several other phyla whose relationships are upheld by genetic evidence.
• Trochophore larvae are distinguished from the lophophores by two bands of cilia around the body; they include the Nemertea, Mollusca, Sipuncula, and Annelida.
• The lophotrochozoans have a mesoderm layer positioned between the ectoderm and endoderm and are bilaterally symmetrical, which signals the beginning of cephalization, the concentration of nervous tissues and sensory organs in the head of the organism.
Key Terms
• blastopore: the opening into the archenteron
• lophophore: a feeding organ of brachiopods, bryozoans, and phoronids
• cephalization: an evolutionary trend in which the neural and sense organs become centralized at one end (the head) of an animal
Lophotrochozoans
Animals belonging to superphylum Lophotrochozoa are protostomes: the blastopore (or the point of involution of the ectoderm or outer germ layer) becomes the mouth opening to the alimentary canal. This is called protostomy or “first mouth.” In protostomy, solid groups of cells split from the endoderm or inner germ layer to form a central mesodermal layer of cells. This layer multiplies into a band which then splits internally to form the coelom; this protostomic coelom is termed schizocoelom.
As lophotrochozoans, the organisms in this superphylum possess either lophophore or trochophore larvae. The exact relationships between the different phyla are not entirely certain. The lophophores include groups that are united by the presence of the lophophore, a set of ciliated tentacles surrounding the mouth. Lophophorata include the flatworms and several other phyla, including the Bryozoa, Entoprocta, Phoronida, and Brachiopoda. These clades are upheld when RNA sequences are compared. Trochophore larvae are characterized by two bands of cilia around the body. Previously, these were treated together as the Trochozoa, together with the arthropods, which do not produce trochophore larvae, but were considered close relatives of the annelids because they are both segmented. However, they show a number of important differences. Arthropods are now placed separately among the Ecdysozoa. The Trochozoa include the Nemertea, Mollusca, Sipuncula, and Annelida.
The lophotrochozoans are triploblastic, possessing an embryonic mesoderm sandwiched between the ectoderm and endoderm found in the diploblastic cnidarians. These phyla are also bilaterally symmetrical: a longitudinal section will divide them into right and left sides that are symmetrical. They also show the beginning of cephalization: the evolution of a concentration of nervous tissues and sensory organs in the head of the organism, which is where it first encounters its environment.
33.3C: Phylum Rotifera
Rotifers are microscopic organisms named for a rotating structure (called the corona) at their anterior end that is covered with cilia.
Learning Objectives
• Identify the features of rotifers involved in movement and feeding
Key Points
• The rotifer body form consists of a head (containing the sensory organs in the form of a bi-lobed brain and small eyespots near the corona), the trunk (containing organs), and the foot (which can hold fast).
• The foot of the rotifer secretes a sticky material to help it adhere to surfaces.
• Rotifers are filter feeders that generate a current using the corona to pass food into the mouth, which then passes by digestive and salivary glands into the stomach and intestines.
• Rotifers exhibit sexual dimorphism; the gender of many species is determined by whether the egg is fertilized (and develops into a female) or unfertilized (and develops into a male).
Key Terms
• pseudocoelomate: any invertebrate animal with a three-layered body and a pseudocoel
• mastax: the pharynx of a rotifer which usually contains four horny pieces that work to crush the food
Phylum Rotifera
The rotifers are a microscopic (about 100 µm to 30 mm) group of mostly-aquatic organisms that get their name from the corona: a rotating, wheel-like structure that is covered with cilia at their anterior end. Although their taxonomy is currently in flux, one treatment places the rotifers in three classes: Bdelloidea, Monogononta, and Seisonidea. The classification of the group is currently under revision, however, as more phylogenetic evidence becomes available. It is possible that the “spiny headed worms” currently in phylum Acanthocephala will be incorporated into this group in the future.
The rotifer body form consists of a head (which contains the corona), a trunk (which contains the organs), and the foot. Rotifers are typically free-swimming and truly planktonic organisms, but the toes or extensions of the foot can secrete a sticky material forming a holdfast to help them adhere to surfaces. The head contains sensory organs in the form of a bi-lobed brain and small eyespots near the corona.
The rotifers are filter feeders that will eat dead material, algae, and other microscopic living organisms. Therefore, they are very important components of aquatic food webs. Rotifers obtain food that is directed toward the mouth by the current created from the movement of the corona. The food particles enter the mouth and travel to the mastax (pharynx with jaw-like structures). Food passes by digestive and salivary glands into the stomach and then into the intestines. Digestive and excretory wastes are collected in a cloacal bladder before being released out the anus.
Rotifers are pseudocoelomates commonly found in fresh water and some salt water environments throughout the world. About 2,200 species of rotifers have been identified. Rotifers are dioecious organisms (having either male or female genitalia) and exhibit sexual dimorphism (males and females have different forms). Many species are parthenogenic and exhibit haplodiploidy, a method of gender determination in which a fertilized egg develops into a female and an unfertilized egg develops into a male. In many dioecious species, males are short-lived and smaller, with no digestive system and a single testis. Females can produce eggs that are capable of dormancy, which protects eggs during harsh environmental conditions. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.03%3A_Rotifers_%28Rotifera%29/33.3A%3A_Superphylum_Lophotrochozoa.txt |
Mollusks have a soft body and share several characteristics, including a muscular foot, a visceral mass of internal organs, and a mantle.
Learning Objectives
• Describe the unique anatomical and morphological features of molluscs
Key Points
• A mollusk’s muscular foot is used for locomotion and anchorage, varies in shape and function, and can both extend and retract.
• The visceral mass inside the mollusk includes digestive, nervous, excretory, reproductive, and respiratory systems.
• Most mollusks possess a radula, which is similar to a tongue with teeth-like projections, serving to shred or scrape food.
• The mantle is the dorsal epidermis in mollusks; in some mollusks it secretes a chitinous and hard calcareous shell.
Key Terms
• visceral mass: the soft, non-muscular metabolic region of the mollusc that contains the body organs
• mantle: the body wall of a mollusc, from which the shell is secreted
• radula: the rasping tongue of snails and most other mollusks
Phylum Mollusca
Phylum Mollusca is the predominant phylum in marine environments. It is estimated that 23 percent of all known marine species are mollusks; there are around 85,000 described species, making them the second most diverse phylum of animals. The name “mollusca” signifies a soft body; the earliest descriptions of mollusks came from observations of unshelled cuttlefish. Mollusks are predominantly a marine group of animals; however, they are known to inhabit freshwater as well as terrestrial habitats. Mollusks display a wide range of morphologies in each class and subclass. They range from large predatory squids and octopus, some of which show a high degree of intelligence, to grazing forms with elaborately-sculpted and colored shells. In spite of their tremendous diversity, however, they also share a few key characteristics, including a muscular foot, a visceral mass containing internal organs, and a mantle that may or may not secrete a shell of calcium carbonate.
Mollusks have a muscular foot used for locomotion and anchorage that varies in shape and function, depending on the type of mollusk under study. In shelled mollusks, this foot is usually the same size as the opening of the shell. The foot is a retractable as well as an extendable organ. It is the ventral-most organ, whereas the mantle is the limiting dorsal organ. Mollusks are eucoelomate, but the cavity is restricted to a region around the heart in adult animals. The mantle cavity develops independently of the coelomic cavity.
The visceral mass is present above the foot in the visceral hump. This includes digestive, nervous, excretory, reproductive, and respiratory systems. Mollusk species that are exclusively aquatic have gills for respiration, whereas some terrestrial species have lungs for respiration. Additionally, a tongue-like organ called a radula, which bears chitinous tooth-like ornamentation, is present in many species, serving to shred or scrape food. The mantle (also known as the pallium) is the dorsal epidermis in mollusks; shelled mollusks are specialized to secrete a chitinous and hard calcareous shell.
Most mollusks are dioecious animals where fertilization occurs externally, although this is not the case in terrestrial mollusks, such as snails and slugs, or in cephalopods. In some mollusks, the zygote hatches and undergoes two larval stages, trochophore and veliger, before becoming a young adult; bivalves may exhibit a third larval stage, glochidia. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.04%3A_Mollusks_%28Mollusca%29/33.4E%3A_Phylum_Mollusca.txt |
The phylum Mollusca includes a wide variety of animals including the gastropods (“stomach foot”), the cephalopods (“head foot”), and the scaphopods (“boat foot”).
Learning Objectives
• Differentiate among the classes in the phylum mollusca
Key Points
• Mollusks can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. These classes are distinguished by, among other criteria, the presence and types of shells they possess.
• Class Aplacophora includes worm-like animals with no shell and a rudimentary body structure.
• Members of class Monoplacophora have a single shell that encloses the body.
• Members of class Polyplacophora are better known as “chitons;” these molluscs have a large foot on the ventral side and a shell composed of eight hard plates on the dorsal side.
• Class Bivalvia consists of mollusks with two shells held together by a muscle; these include oysters, clams, and mussels.
• Members of class Gastropoda have an asymmetrical body plan and usually have a shell, which can be planospiral or conispiral. Their key characteristic is the torsion around the perpendicular axis on the center of the foot that is modified for crawling.
• Class Scaphopoda consists of mollusks with a single conical shell through which the head protrudes, and a foot modified into tentacles known as captaculae that are used to catch and manipulate prey.
Key Terms
• ctenidium: a respiratory system, in the form of a comb, in some molluscs
• captacula: the foot of a Scaphalopod, modified into tentacles for capturing prey
• nephridium: a tubular excretory organ in some invertebrates
Classes in Phylum Mollusca
Phylum Mollusca is a very diverse (85,000 species ) group of mostly marine species, with a dramatic variety of form. This phylum can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda.
Class Aplacophora
Class Aplacophora (“bearing no plates”) includes worm-like animals primarily found in benthic marine habitats. These animals lack a calcareous shell, but possess aragonite spicules on their epidermis. They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia (excretory organs).
Class Monoplacophora
Members of class Monoplacophora (“bearing one plate”) posses a single, cap-like shell that encloses the body. The morphology of the shell and the underlying animal can vary from circular to ovate. A looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads are present in these animals. The monoplacophorans were believed extinct and only known via fossil records until the discovery of Neopilina galathaea in 1952. Today, scientists have identified nearly two dozen extant species.
Class Polyplacophora
Animals in the class Polyplacophora (“bearing many plates”) are commonly known as “chitons” and bear an armor-like, eight-plated dorsal shell. These animals have a broad, ventral foot that is adapted for suction to rocks and other substrates, and a mantle that extends beyond the shell in the form of a girdle. Calcareous spines may be present on the girdle to offer protection from predators. Chitons live worldwide, in cold water, warm water, and the tropics. Most chiton species inhabit intertidal or subtidal zones, and do not extend beyond the photic zone. Some species live quite high in the intertidal zone and are exposed to the air and light for long periods.
Class Bivalvia
Bivalvia is a class of marine and freshwater molluscs with laterally compressed bodies enclosed by a shell in two hinged parts. Bivalves include clams, oysters, mussels, scallops, and numerous other families of shells. The majority are filter feeders and have no head or radula. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment on the seabed, while others lie on the sea floor or attach themselves to rocks or other hard surfaces.
The shell of a bivalve is composed of calcium carbonate, and consists of two, usually similar, parts called valves. These are joined together along one edge by a flexible ligament that, in conjunction with interlocking “teeth” on each of the valves, forms the hinge.
Class Gastropoda
Animals in class Gastropoda (“stomach foot”) include well-known mollusks like snails, slugs, conchs, sea hares, and sea butterflies. Gastropoda includes shell-bearing species as well as species with a reduced shell. These animals are asymmetrical and usually present a coiled shell. Shells may be planospiral (like a garden hose wound up), commonly seen in garden snails, or conispiral (like a spiral staircase), commonly seen in marine conches.
The visceral mass in the shelled species displays torsion around the perpendicular axis on the center of the foot, which is the key characteristic of this group, along with a foot that is modified for crawling. Most gastropods bear a head with tentacles, eyes, and a style. A complex radula is used by the digestive system and aids in the ingestion of food. Eyes may be absent in some gastropods species. The mantle cavity encloses the ctenidia (singluar: ctenidium) as well as a pair of nephridia (singular: nephridium).
Class Cephalopoda
Class Cephalopoda (“head foot” animals) includes octopuses, squids, cuttlefish, and nautilus. Cephalopods are a class of shell-bearing animals as well as mollusks with a reduced shell. They display vivid coloration, typically seen in squids and octopuses which is used for camouflage. All animals in this class are carnivorous predators and have beak-like jaws at the anterior end. All cephalopods show the presence of a very well-developed nervous system along with eyes, as well as a closed circulatory system. The foot is lobed and developed into tentacles and a funnel, which is used as the mode of locomotion. Locomotion in cephalopods is facilitated by ejecting a stream of water for propulsion (“jet” propulsion). Cephalopods, such as squids and octopuses, also produce sepia or a dark ink, which is squirted upon a predator to assist in a quick getaway. Suckers are present on the tentacles in octopuses and squid. Ctenidia are enclosed in a large mantle cavity serviced by blood vessels, each with its own associated heart. The mantle has siphonophores that facilitate exchange of water.
A pair of nephridia is present within the mantle cavity. Sexual dimorphism is seen in this class of animals. Members of a species mate, then the female lays the eggs in a secluded and protected niche. Females of some species care for the eggs for an extended period of time and may end up dying during that time period. Reproduction in cephalopods is different from other mollusks in that the egg hatches to produce a juvenile adult without undergoing the trochophore and veliger larval stages.
Class Scaphopoda
Members of class Scaphopoda (“boat feet”) are known colloquially as “tusk shells” or “tooth shells,” as evident when examining Dentalium, one of the few remaining scaphopod genera. Scaphopods are usually buried in sand with the anterior opening exposed to water. These animals bear a single conical shell, which has both ends open. The head is rudimentary and protrudes out of the posterior end of the shell. These animals do not possess eyes, but they have a radula, as well as a foot modified into tentacles with a bulbous end, known as captaculae. Captaculae serve to catch and manipulate prey. Ctenidia are absent in these animals. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.04%3A_Mollusks_%28Mollusca%29/33.4F%3A_Classification_of_Phylum_Mollusca.txt |
Nemertea, or ribbon worms, are distinguished by their proboscis, used for capturing prey and enclosed in a cavity called a rhynchocoel.
Learning Objectives
• Identify the key features of the Phylum Nemertea
Key Points
• The Nemertini are mostly bottom-dwelling marine organisms, although some are found in freshwater and terrestrial habitats.
• Most nemerteans are carnivores, some are scavengers, and others have evolved relationships with some mollusks that are benefit the Nemertean but do not harm the mollusk.
• Nemerteans vary greatly in size and are bilaterally symmetrical; they are unsegmented and resemble a flat tube which can change morphological presentation in response to environmental cues.
• Nemertini have a simple nervous system comprised of a ring of four nerve masses called “ganglia” at the anterior end between the mouth and the foregut from which paired longitudinal nerve cords emerge and extend to the posterior end.
• Nemertini are mostly sexually dimorphic, fertilizing eggs externally by releasing both eggs and sperm into the water; a larva may develop inside the resulting young worm and devour its tissues before metamorphosing into the adult.
Key Terms
• protonephridia: an invertebrate organ which occurs in pairs and removes metabolic wastes from an animal’s body
• rhynchocoel: a cavity which mostly runs above the midline and ends a little short of the rear of the body of a nemertean and extends or retracts the proboscis
• proboscis: an elongated tube from the head or connected to the mouth, of an animal
Phylum Nemertea
The Nemertea are colloquially known as ribbon worms. Most species of phylum Nemertea are marine (predominantly benthic or bottom dwellers) with an estimated 900 species known. However, nemertini have been recorded in freshwater and terrestrial habitats as well. Most nemerteans are carnivores, feeding on worms, clams, and crustaceans. Some species are scavengers, while other nemertini species, such as Malacobdella grossa, have also evolved commensalistic relationships with some mollusks. Interestingly, nemerteans have almost no predators, two species are sold as fish bait, and some species have devastated commercial fishing of clams and crabs.
Morphology
Ribbon worms vary in size from 1 cm to several meters. They show bilateral symmetry and remarkable contractile properties. Because of their contractility, they can change their morphological presentation in response to environmental cues. Animals in phylum Nemertea also show a flattened morphology: they are flat from front to back, like a flattened tube. In addition, nemertea are soft, unsegmented animals.
A unique characteristic of this phylum is the presence of a proboscis enclosed in a rhynchocoel. The proboscis serves to capture food and may be ornamented with barbs in some species. The rhynchocoel is a fluid-filled cavity that extends from the head to nearly two-thirds of the length of the gut in these animals. The proboscis may be extended or retracted by the retractor muscle attached to the wall of the rhynchocoel.
Metabolism
The nemertini show a very well-developed digestive system. A mouth opening that is ventral to the rhynchocoel leads into the foregut, followed by the intestine. The intestine is present in the form of diverticular pouches which ends in a rectum that opens via an anus. Gonads are interspersed with the intestinal diverticular pouches, opening outwards via genital pores. A circulatory system consists of a closed loop of a pair of lateral blood vessels. The circulatory system is derived from the coelomic cavity of the embryo. Some animals may also have cross-connecting vessels in addition to lateral ones. Although these are called blood vessels, since they are of coelomic origin, the circulatory fluid is colorless. Some species bear hemoglobin as well as yellow or green pigments. The blood vessels are connected to the rhynchocoel. The flow of fluid in these vessels is facilitated by the contraction of muscles in the body wall. A pair of protonephridia, or primitive kidneys, is present in these animals to facilitate osmoregulation. Gaseous exchange occurs through the skin in the nemertini.
Nervous System
Nemertini have a ganglion or “brain” situated at the anterior end between the mouth and the foregut, surrounding the digestive system as well as the rhynchocoel. A ring of four nerve masses called “ganglia” comprises the brain in these animals. Paired longitudinal nerve cords emerge from the brain ganglia, extending to the posterior end. Ocelli or eyespots are present in pairs, in multiples of two in the anterior portion of the body. It is speculated that the eyespots originate from neural tissue and not from the epidermis.
Reproduction
Animals in phylum Nemertea show sexual dimorphism, although freshwater species may be hermaphroditic. Eggs and sperm are released into the water; fertilization occurs externally. The zygote develops into a special kind of nemertean larvae called a planuliform larva. In some nemertine species, another larva specific to the nemertinis, a pilidium, may develop inside the young worm from a series of imaginal discs. This larval form, characteristically shaped like a deerstalker cap, devours tissues from the young worm for survival before metamorphosing into the adult-like morphology. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.05%3A_Ribbon_Worms_%28Nemertea%29.txt |
Annelids include segmented worms, such as leeches and earthworms; they are the most advanced worms as they possess a true coelom.
Learning Objectives
• Describe the morphological and anatomical features of annelids
Key Points
• Annelids are often called “segmented worms” because they possess true segmentation of their bodies, with both internal and external morphological features repeated in each body segment.
• The clitellum is a structure on the anterior portion of the worm that generates mucus to aid in sperm transfer from one worm to another; it also forms a cocoon within which fertilization occurs.
• Most annelids have chitinous hairlike extensions in every segment called chaetae that are anchored in the epidermis, although the number and size of chaetae can vary in the different classes.
• Annelids possess a closed circulatory system, lack a well-developed respiratory system, but have well-developed nervous systems.
• Annelids can either have distinct male and female forms or be hermaphrodites (having both male and female reproductive organs). Earthworms are hermaphrodites and can self-fertilize, but prefer to cross-fertilize if possible.
Key Terms
• clitellum: a glandular swelling in the epidermis of some annelid worms; it secretes a viscous fluid in which the eggs are deposited
• chaeta: a chitinous bristle of an annelid worm
• metamerism: the segmentation of the body into similar discrete units
Phylum Annelida
Phylum Annelida contains the class Polychaeta (the polychaetes) and the class Oligochaeta (the earthworms, leeches, and their relatives). These animals are found in marine, terrestrial, and freshwater habitats, but a presence of water or humidity is a critical factor for their survival, especially in terrestrial habitats. The name of the phylum is derived from the Latin word annellus, which means a small ring. Animals in this phylum show parasitic and commensal symbioses with other species in their habitat. Approximately 16,500 species have been described in phylum Annelida. The phylum includes earthworms, polychaete worms, and leeches. Annelids show protostomic development in embryonic stages and are often called “segmented worms” due to their key characteristic of metamerism, or true segmentation.
Morphology
Annelids display bilateral symmetry and are worm-like in overall morphology. They have a segmented body plan where the internal and external morphological features are repeated in each body segment. Metamerism allows animals to become bigger by adding “compartments,” while making their movement more efficient. This metamerism is thought to arise from identical teloblast cells in the embryonic stage, which develop into identical mesodermal structures. The overall body can be divided into head, body, and pygidium (or tail). The clitellum is a reproductive structure that generates mucus that aids in sperm transfer and gives rise to a cocoon within which fertilization occurs; it appears as a fused band in the anterior third of the animal.
Anatomy
The epidermis is protected by an acellular, external cuticle, but this is much thinner than the cuticle found in the ecdysozoans and does not require periodic shedding for growth. Circular as well as longitudinal muscles are located interior to the epidermis. Chitinous hairlike extensions, anchored in the epidermis and projecting from the cuticle, called setae/chaetae are present in every segment. Annelids show the presence of a true coelom, derived from embryonic mesoderm and protostomy. Hence, they are the most advanced worms. A well-developed and complete digestive system is present in earthworms (oligochaetes) with a mouth, muscular pharynx, esophagus, crop, and gizzard being present. The gizzard leads to the intestine and ends in an anal opening. Each segment is limited by a membranous septum that divides the coelomic cavity into a series of compartments.
Annelids possess a closed circulatory system of dorsal and ventral blood vessels that run parallel to the alimentary canal as well as capillaries that service individual tissues. In addition, these vessels are connected by transverse loops in every segment. These animals lack a well-developed respiratory system; gas exchange occurs across the moist body surface. Excretion is facilitated by a pair of metanephridia (a type of primitive “kidney” that consists of a convoluted tubule and an open, ciliated funnel) that is present in every segment towards the ventral side. Annelids show well-developed nervous systems with a nerve ring of fused ganglia present around the pharynx. The nerve cord is ventral in position, bearing enlarged nodes or ganglia in each segment.
Annelids may be either monoecious, with permanent gonads (as in earthworms and leeches), or dioecious, with temporary or seasonal gonads that develop (as in polychaetes). However, cross-fertilization is preferred in hermaphroditic animals. These animals may also show simultaneous hermaphroditism, participating in simultaneous sperm exchange when they are aligned for copulation.
Earthworms are the most abundant members of the class Oligochaeta, distinguished by the presence of the clitellum as well as few, reduced chaetae (“oligo- = “few”; -chaetae = “hairs”). The number and size of chaetae are greatly diminished in Oligochaeta compared to the polychaetes (poly=many, chaetae = hairs). The many chetae of polychaetes are also arranged within fleshy, flat, paired appendages that protrude from each segment. These parapodia may be specialized for different functions in the polychates. A significant difference between leeches and other annelids is the development of suckers at the anterior and posterior ends and an absence of chaetae. Additionally, the segmentation of the body wall may not correspond to the internal segmentation of the coelomic cavity. This adaptation possibly helps the leeches to elongate when they ingest copious quantities of blood from host vertebrates.
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• Egel als Schneckenparasit 04. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Egel_als_Schneckenparasit_04.JPG. License: Public Domain: No Known Copyright | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.06%3A_Annelids_%28Annelida%29.txt |
Nematodes are parasitic and free-living worms that are able to shed their external cuticle in order to grow.
Learning Objectives
• Describe the features of animals classified in phylum Nematoda
Key Points
• Nematodes are in the same phylogenetic grouping as the arthropods because of the presence of an external cuticle that protects the animal and keeps it from drying out.
• There are an estimated 28,000 species of nematodes, with approximately 16,000 of them being parasitic.
• Nematodes are tubular in shape and are considered pseudocoelomates because of they do not possess a true coelom.
• Nematodes do not have a well-developed excretory system, but do have a complete digestive system.
• Nematodes possess the ability to shed their exoskeleton in order to grow, a process called ecdysis.
Key Terms
• exoskeleton: a hard outer structure that provides both structure and protection to creatures such as insects, Crustacea, and Nematoda
Phylum Nematoda
The Nematoda, similar to most other animal phyla, are triploblastic, possessing an embryonic mesoderm that is sandwiched between the ectoderm and endoderm. They are also bilaterally symmetrical: a longitudinal section will divide them into right and left sides that are symmetrical. Furthermore, the nematodes, or roundworms, possess a pseudocoelom and have both free-living and parasitic forms.
Both the nematodes and arthropods belong to the superphylum Ecdysozoa that is believed to be a clade consisting of all evolutionary descendants from one common ancestor. The name derives from the word ecdysis, which refers to the shedding, or molting, of the exoskeleton. The phyla in this group have a hard cuticle covering their bodies, which must be periodically shed and replaced for them to increase in size.
Phylum Nematoda includes more than 28,000 species with an estimated 16,000 being parasitic in nature. Nematodes are present in all habitats.
Morphology
In contrast with cnidarians, nematodes show a tubular morphology and circular cross-section. These animals are pseudocoelomates; they have a complete digestive system with a distinct mouth and anus. This is in contrast with the cnidarians where only one opening is present (an incomplete digestive system).
The cuticle of Nematodes is rich in collagen and a carbohydrate-protein polymer called chitin. It forms an external “skeleton” outside the epidermis. The cuticle also lines many of the organs internally, including the pharynx and rectum. The epidermis can be either a single layer of cells or a syncytium, which is a multinucleated cell formed from the fusion of uninucleated cells.
The overall morphology of these worms is cylindrical, while the head is radially symmetrical. A mouth opening is present at the anterior end with three or six lips. Teeth occur in some species in the form of cuticle extensions. Some nematodes may present other external modifications such as rings, head shields, or warts. Rings, however, do not reflect true internal body segmentation. The mouth leads to a muscular pharynx and intestine, which leads to a rectum and anal opening at the posterior end. In addition, the muscles of nematodes differ from those of most animals; they have a longitudinal layer only, which accounts for the whip-like motion of their movement.
Excretory System
In nematodes, specialized excretory systems are not well developed. Nitrogenous wastes may be lost by diffusion through the entire body or into the pseudocoelom (body cavity), where they are removed by specialized cells. Regulation of water and salt content of the body is achieved by renette glands, present under the pharynx in marine nematodes.
Nervous system
Most nematodes possess four longitudinal nerve cords that run along the length of the body in dorsal, ventral, and lateral positions. The ventral nerve cord is better developed than the dorsal or lateral cords. All nerve cords fuse at the anterior end, around the pharynx, to form head ganglia, or the “brain” of the worm (taking the form of a ring around the pharynx), as well as at the posterior end to form the tail ganglia. In C. elegans, the nervous system accounts for nearly one-third of the total number of cells in the animal.
Reproduction
Nematodes employ a variety of reproductive strategies that range from monoecious to dioecious to parthenogenic, depending upon the species under consideration. C. elegans is a monoecious species, having development of ova contained in a uterus as well as sperm contained in the spermatheca. The uterus has an external opening known as the vulva. The female genital pore is near the middle of the body, whereas the male’s is at the tip. Specialized structures at the tail of the male keep him in place while he deposits sperm with copulatory spicules. Fertilization is internal with embryonic development beginning very soon after fertilization. The embryo is released from the vulva during the gastrulation stage. The embryonic development stage lasts for 14 hours; development then continues through four successive larval stages with ecdysis between each stage (L1, L2, L3, and L4) ultimately leading to the development of a young male or female adult worm. Adverse environmental conditions such as overcrowding and lack of food can result in the formation of an intermediate larval stage known as the dauer larva. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.08%3A_Roundworms_%28Nematoda%29.txt |
Arthropods are the largest grouping of animals all of which have jointed legs and an exoskeleton made of chitin.
Learning Objectives
• Describe the morphology of arthropoda
Key Points
• Arthropods include the Hexapoda (insects), the Crustacea (lobsters, crabs, and shrimp), the Chelicerata (the spiders and scorpions), and the Myriapoda (the centipedes and millipedes).
• Arthropods have a segmented body plan that contains fused segments divided into regions called tagma.
• Arthropods have an open circulatory system and can use book gills, book lungs, or tracheal tubes for respiration.
Key Terms
• tagma: a specialized grouping of arthropodan segments, such as the head, the thorax, and the abdomen with a common function
• malpighian tubule: a tubule that extends from the alimentary canal to the exterior of the organism, excreting water and wastes in the form of solid nitrogenous compounds
• spiracle: a pore or opening used (especially by spiders and some fish) for breathing
Phylum Arthropoda
The name “arthropoda” means “jointed legs” (in the Greek, “arthros” means “joint” and “podos” means “leg”); it aptly describes the enormous number of invertebrates included in this phylum. Arthropods dominate the animal kingdom with an estimated 85 percent of known species included in this phylum; many arthropods are as yet undocumented. The principal characteristics of all the animals in this phylum are functional segmentation of the body and presence of jointed appendages. Arthropods also show the presence of an exoskeleton made principally of chitin, which is a waterproof, tough polysaccharide. Phylum Arthropoda is the largest phylum in the animal world; insects form the single largest class within this phylum. Arthropods are eucoelomate, protostomic organisms.
Phylum Arthropoda includes animals that have been successful in colonizing terrestrial, aquatic, and aerial habitats. This phylum is further classified into five subphyla: Trilobitomorpha (trilobites, all extinct), Hexapoda (insects and relatives), Myriapoda (millipedes, centipedes, and relatives), Crustaceans (crabs, lobsters, crayfish, isopods, barnacles, and some zooplankton), and Chelicerata (horseshoe crabs, arachnids, scorpions, and daddy longlegs). Trilobites are an extinct group of arthropods found chiefly in the pre-Cambrian Era that are probably most closely related to the Chelicerata. These are identified based on fossil records.
Morphology
A unique feature of animals in the arthropod phylum is the presence of a segmented body and fusion of sets of segments that give rise to functional body regions called tagma. Tagma may be in the form of a head, thorax, and abdomen, or a cephalothorax and abdomen, or a head and trunk. A central cavity, called the hemocoel (or blood cavity), is present; the open circulatory system is regulated by a tubular, or single-chambered, heart. Respiratory systems vary depending on the group of arthropod. Insects and myriapods use a series of tubes (tracheae) that branch through the body, open to the outside through openings called spiracles, and perform gas exchange directly between the cells and air in the tracheae. Other organisms use variants of gills and lungs. Aquatic crustaceans utilize gills, terrestrial chelicerates employ book lungs, and aquatic chelicerates use book gills. The book lungs of arachnids (scorpions, spiders, ticks, and mites) contain a vertical stack of hemocoel wall tissue that somewhat resembles the pages of a book. Between each of the “pages” of tissue is an air space. This allows both sides of the tissue to be in contact with the air at all times, greatly increasing the efficiency of gas exchange. The gills of crustaceans are filamentous structures that exchange gases with the surrounding water.
Groups of arthropods also differ in the organs used for excretion. Crustaceans possess green glands while insects use Malpighian tubules, which work in conjunction with the hindgut to reabsorb water while ridding the body of nitrogenous waste. The cuticle is the covering of an arthropod. It is made up of two layers: the epicuticle, which is a thin, waxy, water-resistant outer layer containing no chitin; and the chitinous procuticle, which is beneath the epicuticle. Chitin is a tough, flexible polysaccharide. In order to grow, the arthropod must shed the exoskeleton during a process called ecdysis (“to strip off”); this is a cumbersome method of growth. During this time, the animal is vulnerable to predation.
33.09: Arthropods (Arthropoda)
The Phylum Arthropoda includes a wide range of species divided into the subphyla: Hexapoda, Crustacea, Myriapoda, and Chelicerata.
Learning Objectives
• Differentiate among the subphylums hexapoda, myriapoda, crustacea, and chelicerata
Key Points
• The Hexapoda include insects; the Crustacea include lobster, crabs, and shrimp; the Myriapoda include centipedes and millipedes; and the Chelicerata include spiders, scorpions.
• The Hexapoda are the largest grouping of Arthropods, containing the more than one million species of insects, having representatives with six legs and one pair of antennae.
• The Myriapoda are terrestrial, prefering humid environments; they have between 10 and 750 legs.
• The Crustacea are primarily aquatic arthropods, but also include terrestrial forms, which have a cephalothorax covered by a carapace.
• The Chelicerata, which includes the spiders, horseshoe crabs, and scorpions, have mouth parts that are fang-like and used for capturing prey.
Key Terms
• cephalothorax: the fused head and thorax of spiders and crustaceans
• forcipule: a modified pincer-like foreleg in centipedes, capable of injecting venom
Subphylum Hexapoda
The name Hexapoda denotes the presence of six legs (three pairs) in these animals, which differentiates them from the number of pairs present in other arthropods. Hexapods are characterized by the presence of a head, thorax, and abdomen, constituting three tagma. The thorax bears the wings as well as six legs in three pairs. Many of the common insects we encounter on a daily basis, including ants, cockroaches, butterflies, and flies, are examples of Hexapoda.
Among the hexapods, the insects are the largest class in terms of species diversity as well as biomass in terrestrial habitats ). Typically, the head bears one pair of sensory antennae, mandibles as mouthparts, a pair of compound eyes, and some ocelli (simple eyes), along with numerous sensory hairs. The thorax bears three pairs of legs (one pair per segment) and two pairs of wings, with one pair each on the second and third thoracic segments. The abdomen usually has eleven segments and bears reproductive apertures. Hexapoda includes insects that are winged (like fruit flies) and wingless (like fleas).
Subphylum Myriapoda
Subphylum Myriapoda includes arthropods with numerous legs. Although the name is hyperbolic in suggesting that myriad legs are present in these invertebrates, the number of legs may vary from 10 to 750. This subphylum includes 13,000 species; the most commonly-found examples are millipedes and centipedes. All myriapods are terrestrial animals, prefering a humid environment.
Myriapods are typically found in moist soils, decaying biological material, and leaf litter. Centipedes, such as Scutigera coleoptrata,are classified as chilopods. These animals bear one pair of legs per segment, mandibles as mouthparts, and are somewhat dorsoventrally flattened. The legs in the first segment are modified to form forcipules (poison claws) that deliver venom to prey such as spiders and cockroaches, as centipedes are predatory. Millipedes bear two pairs of legs per diplosegment, a feature that results from embryonic fusion of adjacent pairs of body segments, are usually rounder in cross-section, and are herbivores or detritivores. Millipedes have visibly more numbers of legs as compared to centipedes, although they do not bear a thousand legs.
Subphylum Crustacea
Crustaceans are the most dominant aquatic arthropods since the total number of marine crustacean species stands at 67,000. However, there are also freshwater and terrestrial crustacean species. Krill, shrimp, lobsters, crabs, and crayfish are all examples of crustaceans. Terrestrial species like the wood lice (Armadillidium spp.) (also called pill bugs, rolly pollies, potato bugs, or isopods) are also crustaceans, although the number of non-aquatic species in this subphylum is relatively low.
Crustaceans possess two pairs of antennae, mandibles as mouthparts, and biramous (“two branched”) appendages: their legs are formed in two parts, as distinct from the uniramous (“one branched”) myriapods and hexapods.
Unlike that of the Hexapoda, the head and thorax of most crustaceans is fused to form a cephalothorax, which is covered by a plate called the carapace, thus producing a body structure of two tagma. Crustaceans have a chitinous exoskeleton that is shed by molting whenever the animal increases in size. The exoskeletons of many species are also infused with calcium carbonate, which makes them even stronger than in other arthropods. Crustaceans have an open circulatory system where blood is pumped into the hemocoel by the dorsally-located heart. Hemocyanin and hemoglobin are the respiratory pigments present in these animals.
Subphylum Chelicerata
This subphylum includes animals such as spiders, scorpions, horseshoe crabs, and sea spiders and is predominantly terrestrial, although some marine species also exist. An estimated 77,000 species, found in almost all habitats, are included in subphylum Chelicerata.
The body of chelicerates may be divided into two parts: prosoma and opisthosoma, which are basically the equivalents of cephalothorax (usually smaller) and abdomen (usually larger). A “head” tagmum is not usually discernible. The phylum derives its name from the first pair of appendages, the chelicerae, which are specialized claw-like or fang-like mouthparts. These animals do not possess antennae. The second pair of appendages is known as pedipalps. In some species, such as sea spiders, an additional pair of appendages, called ovigers, is present between the chelicerae and pedipalps.
Chelicerae are used primarily for feeding, but in spiders, these are often modified into fangs that inject venom into their prey before feeding. Members of this subphylum have an open circulatory system with a heart that pumps blood into the hemocoel. Aquatic species have gills, whereas terrestrial species have either trachea or book lungs for gaseous exchange.
The nervous system in chelicerates consists of a brain and two ventral nerve cords. These animals use external as well as internal fertilization strategies for reproduction, depending upon the species and its habitat. Parental care for the young ranges from absolutely none to relatively-prolonged care.
Contributions and Attributions
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44667/latest...ol11448/latest. License: CC BY: Attribution
• Ecdysozoa. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Ecdysozoa. License: CC BY-SA: Attribution-ShareAlike
• ecdysis. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/ecdysis. License: CC BY-SA: Attribution-ShareAlike
• cuticle. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/cuticle. License: CC BY-SA: Attribution-ShareAlike
• coelomate. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/coelomate. License: CC BY-SA: Attribution-ShareAlike
• Cicada Molting. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...da_Molting.jpg. License: CC BY-SA: Attribution-ShareAlike
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44667/latest...ol11448/latest. License: CC BY: Attribution
• exoskeleton. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/exoskeleton. License: CC BY-SA: Attribution-ShareAlike
• Cicada Molting. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...da_Molting.jpg. License: CC BY-SA: Attribution-ShareAlike
• Soybean cyst nematode and egg SEM. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...nd_egg_SEM.jpg. License: CC BY: Attribution
• malpighian tubule. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/malpighian+tubule. License: CC BY-SA: Attribution-ShareAlike
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44667/latest...ol11448/latest. License: CC BY: Attribution
• spiracle. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/spiracle. License: CC BY-SA: Attribution-ShareAlike
• tagma. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/tagma. License: CC BY-SA: Attribution-ShareAlike
• Cicada Molting. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...da_Molting.jpg. License: CC BY-SA: Attribution-ShareAlike
• Soybean cyst nematode and egg SEM. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...nd_egg_SEM.jpg. License: CC BY: Attribution
• BLW Trilobite (Paradoxides sp.). Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...xides_sp.).jpg. License: CC BY-SA: Attribution-ShareAlike
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• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44667/latest...ol11448/latest. License: CC BY: Attribution
• cephalothorax. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/cephalothorax. License: CC BY-SA: Attribution-ShareAlike
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• Soybean cyst nematode and egg SEM. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...nd_egg_SEM.jpg. License: CC BY: Attribution
• BLW Trilobite (Paradoxides sp.). Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...xides_sp.).jpg. License: CC BY-SA: Attribution-ShareAlike
• Horseshoecrab2. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...eshoecrab2.jpg. License: CC BY: Attribution
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• Carcinus aestuarii 2009 G1. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...ii_2009_G1.jpg. License: CC BY-SA: Attribution-ShareAlike
• Solifugae Chelicera lateral aspect 2012 01 24 0999s. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...1_24_0999s.JPG. License: CC BY: Attribution | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/33%3A_Protostomes/33.09%3A_Arthropods_%28Arthropoda%29/33.9D%3A_Subphyla_of_Arthropoda.txt |
• 34.1: Echinoderms
The phyla Echinodermata and Chordata (the phylum in which humans are placed) both belong to the superphylum Deuterostomia. Recall that protostome and deuterostomes differ in certain aspects of their embryonic development, and they are named based on which opening of the digestive cavity develops first. The word deuterostome comes from the Greek word meaning “mouth second,” indicating that the anus is the first to develop.
• 34.2: Chordates
The phyla Echinodermata and Chordata (the phylum in which humans are placed) both belong to the superphylum Deuterostomia. Recall that protostome and deuterostomes differ in certain aspects of their embryonic development, and they are named based on which opening of the digestive cavity develops first. The word deuterostome comes from the Greek word meaning “mouth second,” indicating that the anus is the first to develop.
• 34.3: Nonvertebrate Chordates
The phyla Echinodermata and Chordata (the phylum in which humans are placed) both belong to the superphylum Deuterostomia. Recall that protostome and deuterostomes differ in certain aspects of their embryonic development, and they are named based on which opening of the digestive cavity develops first. The word deuterostome comes from the Greek word meaning “mouth second,” indicating that the anus is the first to develop.
• 34.4: Vertebrate Chordates
Animals in the phylum Chordata share four key features that appear at some stage during their development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. In some groups, some of these are present only during embryonic development. The chordates are named for the notochord, which is a flexible, rod-shaped structure that is found in the embryonic stage of all chordates and in the adult stage of some chordate species.
• 34.5: Fishes
Modern fishes include an estimated 31,000 species. Fishes were the earliest vertebrates, with jawless species being the earliest and jawed species evolving later. They are active feeders, rather than sessile, suspension feeders. Jawless fishes—the hagfishes and lampreys—have a distinct cranium and complex sense organs including eyes, distinguishing them from the invertebrate chordates.
• 34.6: Amphibians
Amphibians are vertebrate tetrapods. Amphibia includes frogs, salamanders, and caecilians. The term amphibian loosely translates from the Greek as “dual life,” which is a reference to the metamorphosis that many frogs and salamanders undergo and their mixture of aquatic and terrestrial environments in their life cycle. Amphibians evolved during the Devonian period and were the earliest terrestrial tetrapods.
• 34.7: Reptiles
The amniotes —reptiles, birds, and mammals—are distinguished from amphibians by their terrestrially adapted egg, which is protected by amniotic membranes. The evolution of amniotic membranes meant that the embryos of amniotes were provided with their own aquatic environment, which led to less dependence on water for development and thus allowed the amniotes to branch out into drier environments.
• 34.8: Birds
The most obvious characteristic that sets birds apart from other modern vertebrates is the presence of feathers, which are modified scales. While vertebrates like bats fly without feathers, birds rely on feathers and wings, along with other modifications of body structure and physiology, for flight.
• 34.9: Mammals
Mammals are vertebrates that possess hair and mammary glands. Several other characteristics are distinctive to mammals, including certain features of the jaw, skeleton, integument, and internal anatomy. Modern mammals belong to three clades: monotremes, marsupials, and eutherians (or placental mammals).
• 34.10: Evolution of Primates
Order Primates of class Mammalia includes lemurs, tarsiers, monkeys, apes, and humans. Non-human primates live primarily in the tropical or subtropical regions of South America, Africa, and Asia. They range in size from the mouse lemur at 30 grams (1 ounce) to the mountain gorilla at 200 kilograms (441 pounds). The characteristics and evolution of primates is of particular interest to us as it allows us to understand the evolution of our own species.
34: Deuterostomes
Skills to Develop
• Describe the distinguishing characteristics of echinoderms
• Describe the distinguishing characteristics of chordates
The phyla Echinodermata and Chordata (the phylum in which humans are placed) both belong to the superphylum Deuterostomia. Recall that protostome and deuterostomes differ in certain aspects of their embryonic development, and they are named based on which opening of the digestive cavity develops first. The word deuterostome comes from the Greek word meaning “mouth second,” indicating that the anus is the first to develop. There are a series of other developmental characteristics that differ between protostomes and deuterostomes, including the mode of formation of the coelom and the early cell division of the embryo. In deuterostomes, internal pockets of the endodermal lining called the archenteron fuse to form the coelom. The endodermal lining of the archenteron (or the primitive gut) forms membrane protrusions that bud off and become the mesodermal layer. These buds, known as coelomic pouches, fuse to form the coelomic cavity, as they eventually separate from the endodermal layer. The resultant coelom is termed an enterocoelom. The archenteron develops into the alimentary canal, and a mouth opening is formed by invagination of ectoderm at the pole opposite the blastopore of the gastrula. The blastopore forms the anus of the alimentary system in the juvenile and adult forms. The fates of embryonic cells in deuterostomes can be altered if they are experimentally moved to a different location in the embryo due to indeterminant cleavage in early embryogenesis.
Phylum Echinodermata
Echinodermata are so named owing to their spiny skin (from the Greek “echinos” meaning “spiny” and “dermos” meaning “skin”), and this phylum is a collection of about 7,000 described living species. Echinodermata are exclusively marine organisms. Sea stars (Figure \(1\)), sea cucumbers, sea urchins, sand dollars, and brittle stars are all examples of echinoderms. To date, no freshwater or terrestrial echinoderms are known.
Morphology and Anatomy
Adult echinoderms exhibit pentaradial symmetry and have a calcareous endoskeleton made of ossicles, although the early larval stages of all echinoderms have bilateral symmetry. The endoskeleton is developed by epidermal cells and may possess pigment cells, giving vivid colors to these animals, as well as cells laden with toxins. Gonads are present in each arm. In echinoderms like sea stars, every arm bears two rows of tube feet on the oral side. These tube feet help in attachment to the substratum. These animals possess a true coelom that is modified into a unique circulatory system called a water vascular system. An interesting feature of these animals is their power to regenerate, even when over 75 percent of their body mass is lost.
Water Vascular System
Echinoderms possess a unique ambulacral or water vascular system, consisting of a central ring canal and radial canals that extend along each arm. Water circulates through these structures and facilitates gaseous exchange as well as nutrition, predation, and locomotion. The water vascular system also projects from holes in the skeleton in the form of tube feet. These tube feet can expand or contract based on the volume of water present in the system of that arm. By using hydrostatic pressure, the animal can either protrude or retract the tube feet. Water enters the madreporite on the aboral side of the echinoderm. From there, it passes into the stone canal, which moves water into the ring canal. The ring canal connects the radial canals (there are five in a pentaradial animal), and the radial canals move water into the ampullae, which have tube feet through which the water moves. By moving water through the unique water vascular system, the echinoderm can move and force open mollusk shells during feeding.
Nervous System
The nervous system in these animals is a relatively simple structure with a nerve ring at the center and five radial nerves extending outward along the arms. Structures analogous to a brain or derived from fusion of ganglia are not present in these animals.
Excretory System
Podocytes, cells specialized for ultrafiltration of bodily fluids, are present near the center of echinoderms. These podocytes are connected by an internal system of canals to an opening called the madreporite.
Reproduction
Echinoderms are sexually dimorphic and release their eggs and sperm cells into water; fertilization is external. In some species, the larvae divide asexually and multiply before they reach sexual maturity. Echinoderms may also reproduce asexually, as well as regenerate body parts lost in trauma.
Classes of Echinoderms
This phylum is divided into five extant classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lilies or feather stars), and Holothuroidea (sea cucumbers) (Figure \(2\)).
The most well-known echinoderms are members of class Asteroidea, or sea stars. They come in a large variety of shapes, colors, and sizes, with more than 1,800 species known so far. The key characteristic of sea stars that distinguishes them from other echinoderm classes includes thick arms (ambulacra) that extend from a central disk where organs penetrate into the arms. Sea stars use their tube feet not only for gripping surfaces but also for grasping prey. Sea stars have two stomachs, one of which can protrude through their mouths and secrete digestive juices into or onto prey, even before ingestion. This process can essentially liquefy the prey and make digestion easier.
Brittle stars belong to the class Ophiuroidea. Unlike sea stars, which have plump arms, brittle stars have long, thin arms that are sharply demarcated from the central disk. Brittle stars move by lashing out their arms or wrapping them around objects and pulling themselves forward. Sea urchins and sand dollars are examples of Echinoidea. These echinoderms do not have arms, but are hemispherical or flattened with five rows of tube feet that help them in slow movement; tube feet are extruded through pores of a continuous internal shell called a test. Sea lilies and feather stars are examples of Crinoidea. Both of these species are suspension feeders. Sea cucumbers of class Holothuroidea are extended in the oral-aboral axis and have five rows of tube feet. These are the only echinoderms that demonstrate “functional” bilateral symmetry as adults, because the uniquely extended oral-aboral axis compels the animal to lie horizontally rather than stand vertically.
Phylum Chordata
Animals in the phylum Chordata share four key features that appear at some stage of their development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. In some groups, some of these traits are present only during embryonic development. In addition to containing vertebrate classes, the phylum Chordata contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets). Most tunicates live on the ocean floor and are suspension feeders. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms.
Summary
Echinoderms are deuterostomic marine organisms. This phylum of animals bears a calcareous endoskeleton composed of ossicles. These animals also have spiny skin. Echinoderms possess water-based circulatory systems. A pore termed the madreporite is the point of entry and exit for water into the water vascular system. Osmoregulation is carried out by specialized cells known as podocytes.
The characteristic features of Chordata are a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Chordata contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets), together with the vertebrates in Vertebrata. Most tunicates live on the ocean floor and are suspension feeders. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms.
Glossary
archenteron
primitive gut cavity within the gastrula that opens outwards via the blastopore
Chordata
phylum of animals distinguished by their possession of a notochord, a dorsal, hollow nerve cord, pharyngeal slits, and a post-anal tail at some point in their development
Echinodermata
phylum of deuterostomes with spiny skin; exclusively marine organisms
enterocoelom
coelom formed by fusion of coelomic pouches budded from the endodermal lining of the archenteron
madreporite
pore for regulating entry and exit of water into the water vascular system
water vascular system
system in echinoderms where water is the circulatory fluid | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/34%3A_Deuterostomes/34.01%3A_Echinoderms.txt |
Skills to Develop
• Describe the distinguishing characteristics of echinoderms
• Describe the distinguishing characteristics of chordates
The phyla Echinodermata and Chordata (the phylum in which humans are placed) both belong to the superphylum Deuterostomia. Recall that protostome and deuterostomes differ in certain aspects of their embryonic development, and they are named based on which opening of the digestive cavity develops first. The word deuterostome comes from the Greek word meaning “mouth second,” indicating that the anus is the first to develop. There are a series of other developmental characteristics that differ between protostomes and deuterostomes, including the mode of formation of the coelom and the early cell division of the embryo. In deuterostomes, internal pockets of the endodermal lining called the archenteron fuse to form the coelom. The endodermal lining of the archenteron (or the primitive gut) forms membrane protrusions that bud off and become the mesodermal layer. These buds, known as coelomic pouches, fuse to form the coelomic cavity, as they eventually separate from the endodermal layer. The resultant coelom is termed an enterocoelom. The archenteron develops into the alimentary canal, and a mouth opening is formed by invagination of ectoderm at the pole opposite the blastopore of the gastrula. The blastopore forms the anus of the alimentary system in the juvenile and adult forms. The fates of embryonic cells in deuterostomes can be altered if they are experimentally moved to a different location in the embryo due to indeterminant cleavage in early embryogenesis.
Phylum Echinodermata
Echinodermata are so named owing to their spiny skin (from the Greek “echinos” meaning “spiny” and “dermos” meaning “skin”), and this phylum is a collection of about 7,000 described living species. Echinodermata are exclusively marine organisms. Sea stars (Figure \(1\)), sea cucumbers, sea urchins, sand dollars, and brittle stars are all examples of echinoderms. To date, no freshwater or terrestrial echinoderms are known.
Morphology and Anatomy
Adult echinoderms exhibit pentaradial symmetry and have a calcareous endoskeleton made of ossicles, although the early larval stages of all echinoderms have bilateral symmetry. The endoskeleton is developed by epidermal cells and may possess pigment cells, giving vivid colors to these animals, as well as cells laden with toxins. Gonads are present in each arm. In echinoderms like sea stars, every arm bears two rows of tube feet on the oral side. These tube feet help in attachment to the substratum. These animals possess a true coelom that is modified into a unique circulatory system called a water vascular system. An interesting feature of these animals is their power to regenerate, even when over 75 percent of their body mass is lost.
Water Vascular System
Echinoderms possess a unique ambulacral or water vascular system, consisting of a central ring canal and radial canals that extend along each arm. Water circulates through these structures and facilitates gaseous exchange as well as nutrition, predation, and locomotion. The water vascular system also projects from holes in the skeleton in the form of tube feet. These tube feet can expand or contract based on the volume of water present in the system of that arm. By using hydrostatic pressure, the animal can either protrude or retract the tube feet. Water enters the madreporite on the aboral side of the echinoderm. From there, it passes into the stone canal, which moves water into the ring canal. The ring canal connects the radial canals (there are five in a pentaradial animal), and the radial canals move water into the ampullae, which have tube feet through which the water moves. By moving water through the unique water vascular system, the echinoderm can move and force open mollusk shells during feeding.
Nervous System
The nervous system in these animals is a relatively simple structure with a nerve ring at the center and five radial nerves extending outward along the arms. Structures analogous to a brain or derived from fusion of ganglia are not present in these animals.
Excretory System
Podocytes, cells specialized for ultrafiltration of bodily fluids, are present near the center of echinoderms. These podocytes are connected by an internal system of canals to an opening called the madreporite.
Reproduction
Echinoderms are sexually dimorphic and release their eggs and sperm cells into water; fertilization is external. In some species, the larvae divide asexually and multiply before they reach sexual maturity. Echinoderms may also reproduce asexually, as well as regenerate body parts lost in trauma.
Classes of Echinoderms
This phylum is divided into five extant classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lilies or feather stars), and Holothuroidea (sea cucumbers) (Figure \(2\)).
The most well-known echinoderms are members of class Asteroidea, or sea stars. They come in a large variety of shapes, colors, and sizes, with more than 1,800 species known so far. The key characteristic of sea stars that distinguishes them from other echinoderm classes includes thick arms (ambulacra) that extend from a central disk where organs penetrate into the arms. Sea stars use their tube feet not only for gripping surfaces but also for grasping prey. Sea stars have two stomachs, one of which can protrude through their mouths and secrete digestive juices into or onto prey, even before ingestion. This process can essentially liquefy the prey and make digestion easier.
Brittle stars belong to the class Ophiuroidea. Unlike sea stars, which have plump arms, brittle stars have long, thin arms that are sharply demarcated from the central disk. Brittle stars move by lashing out their arms or wrapping them around objects and pulling themselves forward. Sea urchins and sand dollars are examples of Echinoidea. These echinoderms do not have arms, but are hemispherical or flattened with five rows of tube feet that help them in slow movement; tube feet are extruded through pores of a continuous internal shell called a test. Sea lilies and feather stars are examples of Crinoidea. Both of these species are suspension feeders. Sea cucumbers of class Holothuroidea are extended in the oral-aboral axis and have five rows of tube feet. These are the only echinoderms that demonstrate “functional” bilateral symmetry as adults, because the uniquely extended oral-aboral axis compels the animal to lie horizontally rather than stand vertically.
Phylum Chordata
Animals in the phylum Chordata share four key features that appear at some stage of their development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. In some groups, some of these traits are present only during embryonic development. In addition to containing vertebrate classes, the phylum Chordata contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets). Most tunicates live on the ocean floor and are suspension feeders. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms.
Summary
Echinoderms are deuterostomic marine organisms. This phylum of animals bears a calcareous endoskeleton composed of ossicles. These animals also have spiny skin. Echinoderms possess water-based circulatory systems. A pore termed the madreporite is the point of entry and exit for water into the water vascular system. Osmoregulation is carried out by specialized cells known as podocytes.
The characteristic features of Chordata are a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Chordata contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets), together with the vertebrates in Vertebrata. Most tunicates live on the ocean floor and are suspension feeders. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms.
Glossary
archenteron
primitive gut cavity within the gastrula that opens outwards via the blastopore
Chordata
phylum of animals distinguished by their possession of a notochord, a dorsal, hollow nerve cord, pharyngeal slits, and a post-anal tail at some point in their development
Echinodermata
phylum of deuterostomes with spiny skin; exclusively marine organisms
enterocoelom
coelom formed by fusion of coelomic pouches budded from the endodermal lining of the archenteron
madreporite
pore for regulating entry and exit of water into the water vascular system
water vascular system
system in echinoderms where water is the circulatory fluid | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/34%3A_Deuterostomes/34.02%3A_Chordates.txt |
Skills to Develop
• Describe the distinguishing characteristics of echinoderms
• Describe the distinguishing characteristics of chordates
The phyla Echinodermata and Chordata (the phylum in which humans are placed) both belong to the superphylum Deuterostomia. Recall that protostome and deuterostomes differ in certain aspects of their embryonic development, and they are named based on which opening of the digestive cavity develops first. The word deuterostome comes from the Greek word meaning “mouth second,” indicating that the anus is the first to develop. There are a series of other developmental characteristics that differ between protostomes and deuterostomes, including the mode of formation of the coelom and the early cell division of the embryo. In deuterostomes, internal pockets of the endodermal lining called the archenteron fuse to form the coelom. The endodermal lining of the archenteron (or the primitive gut) forms membrane protrusions that bud off and become the mesodermal layer. These buds, known as coelomic pouches, fuse to form the coelomic cavity, as they eventually separate from the endodermal layer. The resultant coelom is termed an enterocoelom. The archenteron develops into the alimentary canal, and a mouth opening is formed by invagination of ectoderm at the pole opposite the blastopore of the gastrula. The blastopore forms the anus of the alimentary system in the juvenile and adult forms. The fates of embryonic cells in deuterostomes can be altered if they are experimentally moved to a different location in the embryo due to indeterminant cleavage in early embryogenesis.
Phylum Echinodermata
Echinodermata are so named owing to their spiny skin (from the Greek “echinos” meaning “spiny” and “dermos” meaning “skin”), and this phylum is a collection of about 7,000 described living species. Echinodermata are exclusively marine organisms. Sea stars (Figure \(1\)), sea cucumbers, sea urchins, sand dollars, and brittle stars are all examples of echinoderms. To date, no freshwater or terrestrial echinoderms are known.
Morphology and Anatomy
Adult echinoderms exhibit pentaradial symmetry and have a calcareous endoskeleton made of ossicles, although the early larval stages of all echinoderms have bilateral symmetry. The endoskeleton is developed by epidermal cells and may possess pigment cells, giving vivid colors to these animals, as well as cells laden with toxins. Gonads are present in each arm. In echinoderms like sea stars, every arm bears two rows of tube feet on the oral side. These tube feet help in attachment to the substratum. These animals possess a true coelom that is modified into a unique circulatory system called a water vascular system. An interesting feature of these animals is their power to regenerate, even when over 75 percent of their body mass is lost.
Water Vascular System
Echinoderms possess a unique ambulacral or water vascular system, consisting of a central ring canal and radial canals that extend along each arm. Water circulates through these structures and facilitates gaseous exchange as well as nutrition, predation, and locomotion. The water vascular system also projects from holes in the skeleton in the form of tube feet. These tube feet can expand or contract based on the volume of water present in the system of that arm. By using hydrostatic pressure, the animal can either protrude or retract the tube feet. Water enters the madreporite on the aboral side of the echinoderm. From there, it passes into the stone canal, which moves water into the ring canal. The ring canal connects the radial canals (there are five in a pentaradial animal), and the radial canals move water into the ampullae, which have tube feet through which the water moves. By moving water through the unique water vascular system, the echinoderm can move and force open mollusk shells during feeding.
Nervous System
The nervous system in these animals is a relatively simple structure with a nerve ring at the center and five radial nerves extending outward along the arms. Structures analogous to a brain or derived from fusion of ganglia are not present in these animals.
Excretory System
Podocytes, cells specialized for ultrafiltration of bodily fluids, are present near the center of echinoderms. These podocytes are connected by an internal system of canals to an opening called the madreporite.
Reproduction
Echinoderms are sexually dimorphic and release their eggs and sperm cells into water; fertilization is external. In some species, the larvae divide asexually and multiply before they reach sexual maturity. Echinoderms may also reproduce asexually, as well as regenerate body parts lost in trauma.
Classes of Echinoderms
This phylum is divided into five extant classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lilies or feather stars), and Holothuroidea (sea cucumbers) (Figure \(2\)).
The most well-known echinoderms are members of class Asteroidea, or sea stars. They come in a large variety of shapes, colors, and sizes, with more than 1,800 species known so far. The key characteristic of sea stars that distinguishes them from other echinoderm classes includes thick arms (ambulacra) that extend from a central disk where organs penetrate into the arms. Sea stars use their tube feet not only for gripping surfaces but also for grasping prey. Sea stars have two stomachs, one of which can protrude through their mouths and secrete digestive juices into or onto prey, even before ingestion. This process can essentially liquefy the prey and make digestion easier.
Brittle stars belong to the class Ophiuroidea. Unlike sea stars, which have plump arms, brittle stars have long, thin arms that are sharply demarcated from the central disk. Brittle stars move by lashing out their arms or wrapping them around objects and pulling themselves forward. Sea urchins and sand dollars are examples of Echinoidea. These echinoderms do not have arms, but are hemispherical or flattened with five rows of tube feet that help them in slow movement; tube feet are extruded through pores of a continuous internal shell called a test. Sea lilies and feather stars are examples of Crinoidea. Both of these species are suspension feeders. Sea cucumbers of class Holothuroidea are extended in the oral-aboral axis and have five rows of tube feet. These are the only echinoderms that demonstrate “functional” bilateral symmetry as adults, because the uniquely extended oral-aboral axis compels the animal to lie horizontally rather than stand vertically.
Phylum Chordata
Animals in the phylum Chordata share four key features that appear at some stage of their development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. In some groups, some of these traits are present only during embryonic development. In addition to containing vertebrate classes, the phylum Chordata contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets). Most tunicates live on the ocean floor and are suspension feeders. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms.
Summary
Echinoderms are deuterostomic marine organisms. This phylum of animals bears a calcareous endoskeleton composed of ossicles. These animals also have spiny skin. Echinoderms possess water-based circulatory systems. A pore termed the madreporite is the point of entry and exit for water into the water vascular system. Osmoregulation is carried out by specialized cells known as podocytes.
The characteristic features of Chordata are a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Chordata contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets), together with the vertebrates in Vertebrata. Most tunicates live on the ocean floor and are suspension feeders. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms.
Glossary
archenteron
primitive gut cavity within the gastrula that opens outwards via the blastopore
Chordata
phylum of animals distinguished by their possession of a notochord, a dorsal, hollow nerve cord, pharyngeal slits, and a post-anal tail at some point in their development
Echinodermata
phylum of deuterostomes with spiny skin; exclusively marine organisms
enterocoelom
coelom formed by fusion of coelomic pouches budded from the endodermal lining of the archenteron
madreporite
pore for regulating entry and exit of water into the water vascular system
water vascular system
system in echinoderms where water is the circulatory fluid | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/34%3A_Deuterostomes/34.03%3A_Nonvertebrate_Chordates.txt |
Skills to Develop
• Describe the distinguishing characteristics of chordates
• Identify the derived character of craniates that sets them apart from other chordates
• Describe the developmental fate of the notochord in vertebrates
Vertebrates are members of the kingdom Animalia and the phylum Chordata (Figure \(1\)). Recall that animals that possess bilateral symmetry can be divided into two groups—protostomes and deuterostomes—based on their patterns of embryonic development. The deuterostomes, whose name translates as “second mouth,” consist of two phyla: Chordata and Echinodermata. Echinoderms are invertebrate marine animals that have pentaradial symmetry and a spiny body covering, a group that includes sea stars, sea urchins, and sea cucumbers. The most conspicuous and familiar members of Chordata are vertebrates, but this phylum also includes two groups of invertebrate chordates.
Characteristics of Chordata
Animals in the phylum Chordata share four key features that appear at some stage during their development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail (Figure \(2\)). In some groups, some of these are present only during embryonic development.
The chordates are named for the notochord, which is a flexible, rod-shaped structure that is found in the embryonic stage of all chordates and in the adult stage of some chordate species. It is located between the digestive tube and the nerve cord, and provides skeletal support through the length of the body. In some chordates, the notochord acts as the primary axial support of the body throughout the animal’s lifetime. In vertebrates, the notochord is present during embryonic development, at which time it induces the development of the neural tube and serves as a support for the developing embryonic body. The notochord, however, is not found in the postnatal stage of vertebrates; at this point, it has been replaced by the vertebral column (that is, the spine).
Art Connection
Which of the following statements about common features of chordates is true?
1. The dorsal hollow nerve cord is part of the chordate central nervous system.
2. In vertebrate fishes, the pharyngeal slits become the gills.
3. Humans are not chordates because humans do not have a tail.
4. Vertebrates do not have a notochord at any point in their development; instead, they have a vertebral column.
The dorsal hollow nerve cord derives from ectoderm that rolls into a hollow tube during development. In chordates, it is located dorsal to the notochord. In contrast, other animal phyla are characterized by solid nerve cords that are located either ventrally or laterally. The nerve cord found in most chordate embryos develops into the brain and spinal cord, which compose the central nervous system.
Pharyngeal slits are openings in the pharynx (the region just posterior to the mouth) that extend to the outside environment. In organisms that live in aquatic environments, pharyngeal slits allow for the exit of water that enters the mouth during feeding. Some invertebrate chordates use the pharyngeal slits to filter food out of the water that enters the mouth. In vertebrate fishes, the pharyngeal slits are modified into gill supports, and in jawed fishes, into jaw supports. In tetrapods, the slits are modified into components of the ear and tonsils. Tetrapod literally means “four-footed,” which refers to the phylogenetic history of various groups that evolved accordingly, even though some now possess fewer than two pairs of walking appendages. Tetrapods include amphibians, reptiles, birds, and mammals.
The post-anal tail is a posterior elongation of the body, extending beyond the anus. The tail contains skeletal elements and muscles, which provide a source of locomotion in aquatic species, such as fishes. In some terrestrial vertebrates, the tail also helps with balance, courting, and signaling when danger is near. In humans, the post-anal tail is vestigial, that is, reduced in size and nonfunctional.
Link to Learning
Click for a video discussing the evolution of chordates and five characteristics that they share.
Chordates and the Evolution of Vertebrates
Chordata also contains two clades of invertebrates: Urochordata and Cephalochordata. Members of these groups also possess the four distinctive features of chordates at some point during their development.
Urochordata
Members of Urochordata are also known as tunicates (Figure \(3\)). The name tunicate derives from the cellulose-like carbohydrate material, called the tunic, which covers the outer body of tunicates. Although adult tunicates are classified as chordates, they do not have a notochord, a dorsal hollow nerve cord, or a post-anal tail, although they do have pharyngeal slits. The larval form, however, possesses all four structures. Most tunicates are hermaphrodites. Tunicate larvae hatch from eggs inside the adult tunicate’s body. After hatching, a tunicate larva swims for a few days until it finds a suitable surface on which it can attach, usually in a dark or shaded location. It then attaches via the head to the surface and undergoes metamorphosis into the adult form, at which point the notochord, nerve cord, and tail disappear.
Most tunicates live a sessile existence on the ocean floor and are suspension feeders. The primary foods of tunicates are plankton and detritus. Seawater enters the tunicate’s body through its incurrent siphon. Suspended material is filtered out of this water by a mucous net (pharyngeal slits) and is passed into the intestine via the action of cilia. The anus empties into the excurrent siphon, which expels wastes and water. Tunicates are found in shallow ocean waters around the world.
Cephalochordata
Members of Cephalochordata possess a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail in the adult stage (Figure \(4\)). The notochord extends into the head, which gives the subphylum its name. Extinct members of this subphylum include Pikaia, which is the oldest known cephalochordate. Pikaia fossils were recovered from the Burgess shales of Canada and dated to the middle of the Cambrian age, making them more than 500 million years old.
Extant members of Cephalochordata are the lancelets, named for their blade-like shape. Lancelets are only a few centimeters long and are usually found buried in sand at the bottom of warm temperate and tropical seas. Like tunicates, they are suspension feeders.
Craniata and Vertebrata
A cranium is a bony, cartilaginous, or fibrous structure surrounding the brain, jaw, and facial bones (Figure \(5\)). Most bilaterally symmetrical animals have a head; of these, those that have a cranium compose the clade Craniata. Craniata includes the hagfishes (Myxini), which have a cranium but lack a backbone, and all of the organisms called “vertebrates.”
Vertebrates are members of the clade Vertebrata. Vertebrates display the four characteristic features of the chordates; however, members of this group also share derived characteristics that distinguish them from invertebrate chordates. Vertebrata is named for the vertebral column, composed of vertebrae, a series of separate bones joined together as a backbone (Figure \(6\)). In adult vertebrates, the vertebral column replaces the notochord, which is only seen in the embryonic stage.
Based on molecular analysis, vertebrates appear to be more closely related to lancelets (cephalochordates) than to tunicates (urochordates) among the invertebrate chordates. This evidence suggests that the cephalochordates diverged from Urochordata and the vertebrates subsequently diverged from the cephalochordates. This hypothesis is further supported by the discovery of a fossil in China from the genus Haikouella. This organism seems to be an intermediate form between cephalochordates and vertebrates. The Haikouella fossils are about 530 million years old and appear similar to modern lancelets. These organisms had a brain and eyes, as do vertebrates, but lack the skull found in craniates.1 This evidence suggests that vertebrates arose during the Cambrian explosion. Recall that the “Cambrian explosion” is the name given to a relatively brief span of time during the Cambrian period during which many animal groups appeared and rapidly diversified. Most modern animal phyla originated during the Cambrian explosion.
Vertebrates are the largest group of chordates, with more than 62,000 living species. Vertebrates are grouped based on anatomical and physiological traits. More than one classification and naming scheme is used for these animals. Here we will consider the traditional groups Agnatha, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia, which constitute classes in the subphylum Vertebrata. Many modern authors classify birds within Reptilia, which correctly reflects their evolutionary heritage. We consider them separately only for convenience. Further, we will consider hagfishes and lampreys together as jawless fishes, the agnathans, although emerging classification schemes separate them into chordate jawless fishes (the hagfishes) and vertebrate jawless fishes (the lampreys).
Animals that possess jaws are known as gnathostomes, which means “jawed mouth.” Gnathostomes include fishes and tetrapods—amphibians, reptiles, birds, and mammals. Tetrapods can be further divided into two groups: amphibians and amniotes. Amniotes are animals whose eggs are adapted for terrestrial living, and this group includes mammals, reptiles, and birds. Amniotic embryos, developing in either an externally shed egg or an egg carried by the female, are provided with a water-retaining environment and are protected by amniotic membranes.
Summary
The characteristic features of Chordata are a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Chordata contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets), together with the vertebrates in Vertebrata. Most tunicates live on the ocean floor and are suspension feeders. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms. Vertebrata is named for the vertebral column, which is a feature of almost all members of this clade.
Art Connections
Figure \(2\): Which of the following statements about common features of chordates is true?
1. The dorsal hollow nerve cord is part of the chordate central nervous system.
2. In vertebrate fishes, the pharyngeal slits become the gills.
3. Humans are not chordates because humans do not have a tail.
4. Vertebrates do not have a notochord at any point in their development; instead, they have a vertebral column.
Answer
A
Footnotes
1. 1 Chen, J. Y., Huang, D. Y., and Li, C. W., “An early Cambrian craniate-like chordate,” Nature 402 (1999): 518–522, doi:10.1038/990080.
Glossary
Cephalochordata
chordate clade whose members possess a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail in the adult stage
Chordata
phylum of animals distinguished by their possession of a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some point during their development
Craniata
clade composed of chordates that possess a cranium; includes Vertebrata together with hagfishes
cranium
bony, cartilaginous, or fibrous structure surrounding the brain, jaw, and facial bones
dorsal hollow nerve cord
hollow, tubular structure derived from ectoderm, which is located dorsal to the notochord in chordates
lancelet
member of Cephalochordata; named for its blade-like shape
notochord
flexible, rod-shaped support structure that is found in the embryonic stage of all chordates and in the adult stage of some chordates
pharyngeal slit
opening in the pharynx
post-anal tail
muscular, posterior elongation of the body extending beyond the anus in chordates
tetrapod
phylogenetic reference to an organism with a four-footed evolutionary history; includes amphibians, reptiles, birds, and mammals
tunicate
sessile chordate that is a member of Urochordata
Urochordata
clade composed of tunicates
vertebral column
series of separate bones joined together as a backbone
Vertebrata
members of the phylum Chordata that possess a backbone | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/34%3A_Deuterostomes/34.04%3A_Vertebrate_Chordates.txt |
Skills to Develop
• Describe the difference between jawless and jawed fishes
• Discuss the distinguishing features of sharks and rays compared to other modern fishes
Modern fishes include an estimated 31,000 species. Fishes were the earliest vertebrates, with jawless species being the earliest and jawed species evolving later. They are active feeders, rather than sessile, suspension feeders. Jawless fishes—the hagfishes and lampreys—have a distinct cranium and complex sense organs including eyes, distinguishing them from the invertebrate chordates.
Jawless Fishes
Jawless fishes are craniates that represent an ancient vertebrate lineage that arose over one half-billion years ago. In the past, the hagfishes and lampreys were classified together as agnathans. Today, hagfishes and lampreys are recognized as separate clades, primarily because lampreys are true vertebrates, whereas hagfishes are not. A defining feature is the lack of paired lateral appendages (fins). Some of the earliest jawless fishes were the ostracoderms (which translates to “shell-skin”). Ostracoderms were vertebrate fishes encased in bony armor, unlike present-day jawless fishes, which lack bone in their scales.
Myxini: Hagfishes
The clade Myxini includes at least 20 species of hagfishes. Hagfishes are eel-like scavengers that live on the ocean floor and feed on dead invertebrates, other fishes, and marine mammals (Figure \(1\)). Hagfishes are entirely marine and are found in oceans around the world, except for the polar regions. A unique feature of these animals is the slime glands beneath the skin that release mucus through surface pores. This mucus allows the hagfish to escape from the grip of predators. Hagfish can also twist their bodies in a knot to feed and sometimes eat carcasses from the inside out.
The skeleton of a hagfish is composed of cartilage, which includes a cartilaginous notochord that runs the length of the body. This notochord provides support to the hagfish’s body. Hagfishes do not replace the notochord with a vertebral column during development, as do true vertebrates.
Petromyzontidae: Lampreys
The clade Petromyzontidae includes approximately 35–40 or more species of lampreys. Lampreys are similar to hagfishes in size and shape; however, lampreys possess some vertebral elements. Lampreys lack paired appendages and bone, as do the hagfishes. As adults, lampreys are characterized by a toothed, funnel-like sucking mouth. Many species have a parasitic stage of their life cycle during which they are ectoparasites of fishes (Figure \(2\)).
Lampreys live primarily in coastal and fresh waters, and have a worldwide distribution, except for in the tropics and polar regions. Some species are marine, but all species spawn in fresh water. Eggs are fertilized externally, and the larvae distinctly differ from the adult form, spending 3 to 15 years as suspension feeders. Once they attain sexual maturity, the adults reproduce and die within days.
Lampreys possess a notochord as adults; however, this notochord is surrounded by a cartilaginous structure called an arcualia, which may resemble an evolutionarily early form of the vertebral column.
Gnathostomes: Jawed Fishes
Gnathostomes or “jaw-mouths” are vertebrates that possess jaws. One of the most significant developments in early vertebrate evolution was the development of the jaw, which is a hinged structure attached to the cranium that allows an animal to grasp and tear its food. The evolution of jaws allowed early gnathostomes to exploit food resources that were unavailable to jawless fishes.
Early gnathostomes also possessed two sets of paired fins, allowing the fishes to maneuver accurately. Pectoral fins are typically located on the anterior body, and pelvic fins on the posterior. Evolution of the jaw and paired fins permitted gnathostomes to expand from the sedentary suspension feeding of jawless fishes to become mobile predators. The ability of gnathostomes to exploit new nutrient sources likely is one reason that they replaced most jawless fishes during the Devonian period. Two early groups of gnathostomes were the acanthodians and placoderms (Figure \(3\)), which arose in the late Silurian period and are now extinct. Most modern fishes are gnathostomes that belong to the clades Chondrichthyes and Osteichthyes.
Chondrichthyes: Cartilaginous Fishes
The clade Chondrichthyes is diverse, consisting of sharks (Figure \(4\)), rays, and skates, together with sawfishes and a few dozen species of fishes called chimaeras, or “ghost” sharks.” Chondrichthyes are jawed fishes that possess paired fins and a skeleton made of cartilage. This clade arose approximately 370 million years ago in the early or middle Devonian. They are thought to be descended from the placoderms, which had skeletons made of bone; thus, the cartilaginous skeleton of Chondrichthyes is a later development. Parts of shark skeleton are strengthened by granules of calcium carbonate, but this is not the same as bone.
Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for a part or all of their lives. Most sharks are carnivores that feed on live prey, either swallowing it whole or using their jaws and teeth to tear it into smaller pieces. Shark teeth likely evolved from the jagged scales that cover their skin, called placoid scales. Some species of sharks and rays are suspension feeders that feed on plankton.
Sharks have well-developed sense organs that aid them in locating prey, including a keen sense of smell and electroreception, with the latter perhaps the most sensitive of any animal. Organs called ampullae of Lorenzini allow sharks to detect the electromagnetic fields that are produced by all living things, including their prey. Electroreception has only been observed in aquatic or amphibious animals. Sharks, together with most fishes and aquatic and larval amphibians, also have a sense organ called the lateral line, which is used to detect movement and vibration in the surrounding water, and is often considered homologous to “hearing” in terrestrial vertebrates. The lateral line is visible as a darker stripe that runs along the length of a fish’s body.
Sharks reproduce sexually, and eggs are fertilized internally. Most species are ovoviviparous: The fertilized egg is retained in the oviduct of the mother’s body and the embryo is nourished by the egg yolk. The eggs hatch in the uterus, and young are born alive and fully functional. Some species of sharks are oviparous: They lay eggs that hatch outside of the mother’s body. Embryos are protected by a shark egg case or “mermaid’s purse” (Figure \(5\)) that has the consistency of leather. The shark egg case has tentacles that snag in seaweed and give the newborn shark cover. A few species of sharks are viviparous: The young develop within the mother’s body and she gives live birth.
Rays and skates comprise more than 500 species and are closely related to sharks. They can be distinguished from sharks by their flattened bodies, pectoral fins that are enlarged and fused to the head, and gill slits on their ventral surface (Figure \(6\)). Like sharks, rays and skates have a cartilaginous skeleton. Most species are marine and live on the sea floor, with nearly a worldwide distribution.
Osteichthyes: Bony Fishes
Members of the clade Osteichthyes, also called bony fishes, are characterized by a bony skeleton. The vast majority of present-day fishes belong to this group, which consists of approximately 30,000 species, making it the largest class of vertebrates in existence today.
Nearly all bony fishes have an ossified skeleton with specialized bone cells (osteocytes) that produce and maintain a calcium phosphate matrix. This characteristic has only reversed in a few groups of Osteichthyes, such as sturgeons and paddlefish, which have primarily cartilaginous skeletons. The skin of bony fishes is often covered by overlapping scales, and glands in the skin secrete mucus that reduces drag when swimming and aids the fish in osmoregulation. Like sharks, bony fishes have a lateral line system that detects vibrations in water.
All bony fishes use gills to breathe. Water is drawn over gills that are located in chambers covered and ventilated by a protective, muscular flap called the operculum. Many bony fishes also have a swim bladder, a gas-filled organ that helps to control the buoyancy of the fish. Bony fishes are further divided into two extant clades: Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes).
Actinopterygii, the ray-finned fishes, include many familiar fishes—tuna, bass, trout, and salmon (Figure \(7\)), among others. Ray-finned fishes are named for their fins that are webs of skin supported by bony spines called rays. In contrast, the fins of Sarcopterygii are fleshy and lobed, supported by bone (Figure \(7\)). Living members of this clade include the less-familiar lungfishes and coelacanths.
Summary
The earliest vertebrates that diverged from the invertebrate chordates were the jawless fishes. Fishes with jaws (gnathostomes) evolved later. Jaws allowed early gnathostomes to exploit new food sources. Agnathans include the hagfishes and lampreys. Hagfishes are eel-like scavengers that feed on dead invertebrates and other fishes. Lampreys are characterized by a toothed, funnel-like sucking mouth, and most species are parasitic on other fishes. Gnathostomes include the cartilaginous fishes and the bony fishes, as well as all other tetrapods. Cartilaginous fishes include sharks, rays, skates, and ghost sharks. Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for part or all of their lives. The vast majority of present-day fishes belong to the clade Osteichthyes, which consists of approximately 30,000 species. Bony fishes can be divided into two clades: Actinopterygii (ray-finned fishes, virtually all extant species) and Sarcopterygii (lobe-finned fishes, comprising fewer than 10 extant species but which are the ancestors of tetrapods).
Glossary
Actinopterygii
ray-finned fishes
ampulla of Lorenzini
sensory organ that allows sharks to detect electromagnetic fields produced by living things
Chondrichthyes
jawed fish with paired fins and a skeleton made of cartilage
gnathostome
jawed fish
hagfish
eel-like jawless fish that live on the ocean floor and are scavengers
lamprey
jawless fish characterized by a toothed, funnel-like, sucking mouth
lateral line
sense organ that runs the length of a fish’s body; used to detect vibration in the water
Myxini
hagfishes
Osteichthyes
bony fish
ostracoderm
one of the earliest jawless fish covered in bone
Petromyzontidae
clade of lampreys
Sarcopterygii
lobe-finned fish
swim bladder
in fishes, a gas filled organ that helps to control the buoyancy of the fish | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/34%3A_Deuterostomes/34.05%3A_Fishes.txt |
Skills to Develop
• Describe the important difference between the life cycle of amphibians and the life cycles of other vertebrates
• Distinguish between the characteristics of Urodela, Anura, and Apoda
• Describe the evolutionary history of amphibians
Amphibians are vertebrate tetrapods. Amphibia includes frogs, salamanders, and caecilians. The term amphibian loosely translates from the Greek as “dual life,” which is a reference to the metamorphosis that many frogs and salamanders undergo and their mixture of aquatic and terrestrial environments in their life cycle. Amphibians evolved during the Devonian period and were the earliest terrestrial tetrapods.
Characteristics of Amphibians
As tetrapods, most amphibians are characterized by four well-developed limbs. Some species of salamanders and all caecilians are functionally limbless; their limbs are vestigial. An important characteristic of extant amphibians is a moist, permeable skin that is achieved via mucus glands that keep the skin moist; thus, exchange of oxygen and carbon dioxide with the environment can take place through it (cutaneous respiration). Additional characteristics of amphibians include pedicellate teeth—teeth in which the root and crown are calcified, separated by a zone of noncalcified tissue—and a papilla amphibiorum and papilla basilaris, structures of the inner ear that are sensitive to frequencies below and above 10,00 hertz, respectively. Amphibians also have an auricular operculum, which is an extra bone in the ear that transmits sounds to the inner ear. All extant adult amphibians are carnivorous, and some terrestrial amphibians have a sticky tongue that is used to capture prey.
Evolution of Amphibians
The fossil record provides evidence of the first tetrapods: now-extinct amphibian species dating to nearly 400 million years ago. Evolution of tetrapods from fishes represented a significant change in body plan from one suited to organisms that respired and swam in water, to organisms that breathed air and moved onto land; these changes occurred over a span of 50 million years during the Devonian period. One of the earliest known tetrapods is from the genus Acanthostega. Acanthostega was aquatic; fossils show that it had gills similar to fishes. However, it also had four limbs, with the skeletal structure of limbs found in present-day tetrapods, including amphibians. Therefore, it is thought that Acanthostega lived in shallow waters and was an intermediate form between lobe-finned fishes and early, fully terrestrial tetrapods. What preceded Acanthostega?
In 2006, researchers published news of their discovery of a fossil of a “tetrapod-like fish,” Tiktaalik roseae, which seems to be an intermediate form between fishes having fins and tetrapods having limbs (Figure \(1\)). Tiktaalik likely lived in a shallow water environment about 375 million years ago.1
The early tetrapods that moved onto land had access to new nutrient sources and relatively few predators. This led to the widespread distribution of tetrapods during the early Carboniferous period, a period sometimes called the “age of the amphibians.”
Modern Amphibians
Amphibia comprises an estimated 6,770 extant species that inhabit tropical and temperate regions around the world. Amphibians can be divided into three clades: Urodela (“tailed-ones”), the salamanders; Anura (“tail-less ones”), the frogs; and Apoda (“legless ones”), the caecilians.
Urodela: Salamanders
Salamanders are amphibians that belong to the order Urodela. Living salamanders (Figure \(1\)) include approximately 620 species, some of which are aquatic, other terrestrial, and some that live on land only as adults. Adult salamanders usually have a generalized tetrapod body plan with four limbs and a tail. They move by bending their bodies from side to side, called lateral undulation, in a fish-like manner while “walking” their arms and legs fore and aft. It is thought that their gait is similar to that used by early tetrapods. Respiration differs among different species. The majority of salamanders are lungless, and respiration occurs through the skin or through external gills. Some terrestrial salamanders have primitive lungs; a few species have both gills and lungs.
Unlike frogs, virtually all salamanders rely on internal fertilization of the eggs. The only male amphibians that possess copulatory structures are the caecilians, so fertilization among salamanders typically involves an elaborate and often prolonged courtship. Such a courtship allows the successful transfer of sperm from male to female via a spermatophore. Development in many of the most highly evolved salamanders, which are fully terrestrial, occurs during a prolonged egg stage, with the eggs guarded by the mother. During this time, the gilled larval stage is found only within the egg capsule, with the gills being resorbed, and metamorphosis being completed, before hatching. Hatchlings thus resemble tiny adults.
Link to Learning
View River Monsters: Fish With Arms and Hands? to see a video about an unusually large salamander species.
Anura: Frogs
Frogs are amphibians that belong to the order Anura (Figure \(3\)). Anurans are among the most diverse groups of vertebrates, with approximately 5,965 species that occur on all of the continents except Antarctica. Anurans have a body plan that is more specialized for movement. Adult frogs use their hind limbs to jump on land. Frogs have a number of modifications that allow them to avoid predators, including skin that acts as camouflage. Many species of frogs and salamanders also release defensive chemicals from glands in the skin that are poisonous to predators.
Frog eggs are fertilized externally, and like other amphibians, frogs generally lay their eggs in moist environments. A moist environment is required as eggs lack a shell and thus dehydrate quickly in dry environments. Frogs demonstrate a great diversity of parental behaviors, with some species laying many eggs and exhibiting little parental care, to species that carry eggs and tadpoles on their hind legs or backs. The life cycle of frogs, as other amphibians, consists of two distinct stages: the larval stage followed by metamorphosis to an adult stage. The larval stage of a frog, the tadpole, is often a filter-feeding herbivore. Tadpoles usually have gills, a lateral line system, long-finned tails, and lack limbs. At the end of the tadpole stage, frogs undergo metamorphosis into the adult form (Figure \(4\)). During this stage, the gills, tail, and lateral line system disappear, and four limbs develop. The jaws become larger and are suited for carnivorous feeding, and the digestive system transforms into the typical short gut of a predator. An eardrum and air-breathing lungs also develop. These changes during metamorphosis allow the larvae to move onto land in the adult stage.
Apoda: Caecilians
An estimated 185 species comprise caecilians, a group of amphibians that belong to the order Apoda. Although they are vertebrates, a complete lack of limbs leads to their resemblance to earthworms in appearance. They are adapted for a soil-burrowing or aquatic lifestyle, and they are nearly blind. These animals are found in the tropics of South America, Africa, and Southern Asia. They have vestigial limbs, evidence that they evolved from a legged ancestor.
Evolution Connection: The Paleozoic Era and the Evolution of Vertebrates
The climate and geography of Earth was vastly different during the Paleozoic Era, when vertebrates arose, as compared to today. The Paleozoic spanned from approximately 542 to 251 million years ago. The landmasses on Earth were very different from those of today. Laurentia and Gondwana were continents located near the equator that subsumed much of the current day landmasses in a different configuration (Figure \(5\)). At this time, sea levels were very high, probably at a level that hasn’t been reached since. As the Paleozoic progressed, glaciations created a cool global climate, but conditions warmed near the end of the first half of the Paleozoic. During the latter half of the Paleozoic, the landmasses began moving together, with the initial formation of a large northern block called Laurasia. This contained parts of what is now North America, along with Greenland, parts of Europe, and Siberia. Eventually, a single supercontinent, called Pangaea, was formed, starting in the latter third of the Paleozoic. Glaciations then began to affect Pangaea’s climate, affecting the distribution of vertebrate life.
During the early Paleozoic, the amount of carbon dioxide in the atmosphere was much greater than it is today. This may have begun to change later, as land plants became more common. As the roots of land plants began to infiltrate rock and soil began to form, carbon dioxide was drawn out of the atmosphere and became trapped in the rock. This reduced the levels of carbon dioxide and increased the levels of oxygen in the atmosphere, so that by the end of the Paleozoic, atmospheric conditions were similar to those of today.
As plants became more common through the latter half of the Paleozoic, microclimates began to emerge and ecosystems began to change. As plants and ecosystems continued to grow and become more complex, vertebrates moved from the water to land. The presence of shoreline vegetation may have contributed to the movement of vertebrates onto land. One hypothesis suggests that the fins of aquatic vertebrates were used to maneuver through this vegetation, providing a precursor to the movement of fins on land and the development of limbs. The late Paleozoic was a time of diversification of vertebrates, as amniotes emerged and became two different lines that gave rise, on one hand, to mammals, and, on the other hand, to reptiles and birds. Many marine vertebrates became extinct near the end of the Devonian period, which ended about 360 million years ago, and both marine and terrestrial vertebrates were decimated by a mass extinction in the early Permian period about 250 million years ago.
Link to Learning
View Earth’s Paleogeography: Continental Movements Through Time to see changes in Earth as life evolved.
Summary
As tetrapods, most amphibians are characterized by four well-developed limbs, although some species of salamanders and all caecilians are limbless. The most important characteristic of extant amphibians is a moist, permeable skin used for cutaneous respiration. The fossil record provides evidence of amphibian species, now extinct, that arose over 400 million years ago as the first tetrapods. Amphibia can be divided into three clades: salamanders (Urodela), frogs (Anura), and caecilians (Apoda). The life cycle of frogs, like the majority of amphibians, consists of two distinct stages: the larval stage and metamorphosis to an adult stage. Some species in all orders bypass a free-living larval stage.
Footnotes
1. 1 Daeschler, E. B., Shubin, N. H., and Jenkins, F. J. “A Devonian tetrapod-like fish and the evolution of the tetrapod body plan,” Nature 440 (2006): 757–763, doi:10.1038/nature04639, http://www.nature.com/nature/journal/v440/n7085/abs/nature04639.html.
Glossary
Acanthostega
one of the earliest known tetrapods
Amphibia
frogs, salamanders, and caecilians
Anura
frogs
Apoda
caecilians
caecilian
legless amphibian that belongs to the clade Apoda
cutaneous respiration
gas exchange through the skin
frog
tail-less amphibian that belongs to the clade Anura
salamander
tailed amphibian that belongs to the clade Urodela
tadpole
larval stage of a frog
Urodela
salamanders | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/34%3A_Deuterostomes/34.06%3A_Amphibians.txt |
Skills to Develop
• Describe the main characteristics of amniotes
• Explain the difference between anapsids, synapsids, and diapsids, and give an example of each
• Identify the characteristics of reptiles
• Discuss the evolution of reptiles
The amniotes —reptiles, birds, and mammals—are distinguished from amphibians by their terrestrially adapted egg, which is protected by amniotic membranes. The evolution of amniotic membranes meant that the embryos of amniotes were provided with their own aquatic environment, which led to less dependence on water for development and thus allowed the amniotes to branch out into drier environments. This was a significant development that distinguished them from amphibians, which were restricted to moist environments due their shell-less eggs. Although the shells of various amniotic species vary significantly, they all allow retention of water. The shells of bird eggs are composed of calcium carbonate and are hard, but fragile. The shells of reptile eggs are leathery and require a moist environment. Most mammals do not lay eggs (except for monotremes). Instead, the embryo grows within the mother’s body; however, even with this internal gestation, amniotic membranes are still present.
Characteristics of Amniotes
The amniotic egg is the key characteristic of amniotes. In amniotes that lay eggs, the shell of the egg provides protection for the developing embryo while being permeable enough to allow for the exchange of carbon dioxide and oxygen. The albumin, or egg white, provides the embryo with water and protein, whereas the fattier egg yolk is the energy supply for the embryo, as is the case with the eggs of many other animals, such as amphibians. However, the eggs of amniotes contain three additional extra-embryonic membranes: the chorion, amnion, and allantois (Figure \(1\)). Extra-embryonic membranes are membranes present in amniotic eggs that are not a part of the body of the developing embryo. While the inner amniotic membrane surrounds the embryo itself, the chorion surrounds the embryo and yolk sac. The chorion facilitates exchange of oxygen and carbon dioxide between the embryo and the egg’s external environment. The amnion protects the embryo from mechanical shock and supports hydration. The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration. In mammals, membranes that are homologous to the extra-embryonic membranes in eggs are present in the placenta.
Art Connection
Which of the following statements about the parts of an egg are false?
1. The allantois stores nitrogenous waste and facilitates respiration.
2. The chorion facilitates gas exchange.
3. The yolk provides food for the growing embryo.
4. The amniotic cavity is filled with albumen.
Additional derived characteristics of amniotes include waterproof skin, due to the presence of lipids, and costal (rib) ventilation of the lungs.
Evolution of Amniotes
The first amniotes evolved from amphibian ancestors approximately 340 million years ago during the Carboniferous period. The early amniotes diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids and sauropsids. Synapsids include all mammals, including extinct mammalian species. Synapsids also include therapsids, which were mammal-like reptiles from which mammals evolved. Sauropsids include reptiles and birds, and can be further divided into anapsids and diapsids. The key differences between the synapsids, anapsids, and diapsids are the structures of the skull and the number of temporal fenestrae behind each eye (Figure \(2\)). Temporal fenestrae are post-orbital openings in the skull that allow muscles to expand and lengthen. Anapsids have no temporal fenestrae, synapsids have one, and diapsids have two. Anapsids include extinct organisms and may, based on anatomy, include turtles. However, this is still controversial, and turtles are sometimes classified as diapsids based on molecular evidence. The diapsids include birds and all other living and extinct reptiles.
The diapsids diverged into two groups, the Archosauromorpha (“ancient lizard form”) and the Lepidosauromorpha (“scaly lizard form”) during the Mesozoic period (Figure \(3\)). The lepidosaurs include modern lizards, snakes, and tuataras. The archosaurs include modern crocodiles and alligators, and the extinct pterosaurs (“winged lizard”) and dinosaurs (“terrible lizard”). Clade Dinosauria includes birds, which evolved from a branch of dinosaurs.
Art Connection
Members of the order Testudines have an anapsid-like skull with one opening. However, molecular studies indicate that turtles descended from a diapsid ancestor. Why might this be the case?
In the past, the most common division of amniotes has been into the classes Mammalia, Reptilia, and Aves. Birds are descended, however, from dinosaurs, so this classical scheme results in groups that are not true clades. We will consider birds as a group distinct from reptiles for the purpose of this discussion with the understanding that this does not completely reflect phylogenetic history and relationships.
Characteristics of Reptiles
Reptiles are tetrapods. Limbless reptiles—snakes and other squamates—have vestigial limbs and, like caecilians, are classified as tetrapods because they are descended from four-limbed ancestors. Reptiles lay eggs enclosed in shells on land. Even aquatic reptiles return to the land to lay eggs. They usually reproduce sexually with internal fertilization. Some species display ovoviviparity, with the eggs remaining in the mother’s body until they are ready to hatch. Other species are viviparous, with the offspring born alive.
One of the key adaptations that permitted reptiles to live on land was the development of their scaly skin, containing the protein keratin and waxy lipids, which reduced water loss from the skin. This occlusive skin means that reptiles cannot use their skin for respiration, like amphibians, and thus all breathe with lungs.
Reptiles are ectotherms, animals whose main source of body heat comes from the environment. This is in contrast to endotherms, which use heat produced by metabolism to regulate body temperature. In addition to being ectothermic, reptiles are categorized as poikilotherms, or animals whose body temperatures vary rather than remain stable. Reptiles have behavioral adaptations to help regulate body temperature, such as basking in sunny places to warm up and finding shady spots or going underground to cool down. The advantage of ectothermy is that metabolic energy from food is not required to heat the body; therefore, reptiles can survive on about 10 percent of the calories required by a similarly sized endotherm. In cold weather, some reptiles such as the garter snake brumate. Brumation is similar to hibernation in that the animal becomes less active and can go for long periods without eating, but differs from hibernation in that brumating reptiles are not asleep or living off fat reserves. Rather, their metabolism is slowed in response to cold temperatures, and the animal is very sluggish.
Evolution of Reptiles
Reptiles originated approximately 300 million years ago during the Carboniferous period. One of the oldest known amniotes is Casineria, which had both amphibian and reptilian characteristics. One of the earliest undisputed reptiles was Hylonomus. Soon after the first amniotes appeared, they diverged into three groups—synapsids, anapsids, and diapsids—during the Permian period. The Permian period also saw a second major divergence of diapsid reptiles into archosaurs (predecessors of crocodilians and dinosaurs) and lepidosaurs (predecessors of snakes and lizards). These groups remained inconspicuous until the Triassic period, when the archosaurs became the dominant terrestrial group due to the extinction of large-bodied anapsids and synapsids during the Permian-Triassic extinction. About 250 million years ago, archosaurs radiated into the dinosaurs and the pterosaurs.
Although they are sometimes mistakenly called dinosaurs, the pterosaurs were distinct from true dinosaurs (Figure \(4\)). Pterosaurs had a number of adaptations that allowed for flight, including hollow bones (birds also exhibit hollow bones, a case of convergent evolution). Their wings were formed by membranes of skin that attached to the long, fourth finger of each arm and extended along the body to the legs.
The dinosaurs were a diverse group of terrestrial reptiles with more than 1,000 species identified to date. Paleontologists continue to discover new species of dinosaurs. Some dinosaurs were quadrupeds (Figure \(5\)); others were bipeds. Some were carnivorous, whereas others were herbivorous. Dinosaurs laid eggs, and a number of nests containing fossilized eggs have been found. It is not known whether dinosaurs were endotherms or ectotherms. However, given that modern birds are endothermic, the dinosaurs that served as ancestors to birds likely were endothermic as well. Some fossil evidence exists for dinosaurian parental care, and comparative biology supports this hypothesis since the archosaur birds and crocodilians display parental care.
Dinosaurs dominated the Mesozoic Era, which was known as the “age of reptiles.” The dominance of dinosaurs lasted until the end of the Cretaceous, the last period of the Mesozoic Era. The Cretaceous-Tertiary extinction resulted in the loss of most of the large-bodied animals of the Mesozoic Era. Birds are the only living descendants of one of the major clades of dinosaurs.
Link to Learning
Visit this site to see a video discussing the hypothesis that an asteroid caused the Cretaceous-Triassic (KT) extinction.
Modern Reptiles
Class Reptilia includes many diverse species that are classified into four living clades. These are the 25 species of Crocodilia, 2 species of Sphenodontia, approximately 9,200 Squamata species, and the Testudines, with about 325 species.
Crocodilia
Crocodilia (“small lizard”) arose with a distinct lineage by the middle Triassic; extant species include alligators, crocodiles, and caimans. Crocodilians (Figure \(6\)) live throughout the tropics and subtropics of Africa, South America, Southern Florida, Asia, and Australia. They are found in freshwater, saltwater, and brackish habitats, such as rivers and lakes, and spend most of their time in water. Some species are able to move on land due to their semi-erect posture.
Sphenodontia
Sphenodontia (“wedge tooth”) arose in the Mesozoic era and includes only one living genus, Tuatara, comprising two species that are found in New Zealand (Figure \(7\)). Tuataras measure up to 80 centimeters and weigh about 1 kilogram. Although quite lizard-like in gross appearance, several unique features of the skull and jaws clearly define them and distinguish the group from the squamates.
Squamata
Squamata (“scaly”) arose in the late Permian, and extant species include lizards and snakes. Both are found on all continents except Antarctica. Lizards and snakes are most closely related to tuataras, both groups having evolved from a lepidosaurian ancestor. Squamata is the largest extant clade of reptiles (Figure \(8\)). Most lizards differ from snakes by having four limbs, although these have been variously lost or significantly reduced in at least 60 lineages. Snakes lack eyelids and external ears, which are present in lizards. Lizard species range in size from chameleons and geckos, which are a few centimeters in length, to the Komodo dragon, which is about 3 meters in length. Most lizards are carnivorous, but some large species, such as iguanas, are herbivores.
Snakes are thought to have descended from either burrowing lizards or aquatic lizards over 100 million years ago (Figure \(9\)). Snakes comprise about 3,000 species and are found on every continent except Antarctica. They range in size from 10 centimeter-long thread snakes to 10 meter-long pythons and anacondas. All snakes are carnivorous and eat small animals, birds, eggs, fish, and insects. The snake body form is so specialized that, in its general morphology, a “snake is a snake.” Their specializations all point to snakes having evolved to feed on relatively large prey (even though some current species have reversed this trend). Although variations exist, most snakes have a skull that is very flexible, involving eight rotational joints. They also differ from other squamates by having mandibles (lower jaws) without either bony or ligamentous attachment anteriorly. Having this connection via skin and muscle allows for great expansion of the gape and independent motion of the two sides—both advantages in swallowing big items.
Testudines
Turtles are members of the clade Testudines (“having a shell”) (Figure \(10\)). Turtles are characterized by a bony or cartilaginous shell. The shell consists of the ventral surface called the plastron and the dorsal surface called the carapace, which develops from the ribs. The plastron is made of scutes or plates; the scutes can be used to differentiate species of turtles. The two clades of turtles are most easily recognized by how they retract their necks. The dominant group, which includes all North American species, retracts its neck in a vertical S-curve. Turtles in the less speciose clade retract the neck with a horizontal curve.
Turtles arose approximately 200 million years ago, predating crocodiles, lizards, and snakes. Similar to other reptiles, turtles are ectotherms. They lay eggs on land, although many species live in or near water. None exhibit parental care. Turtles range in size from the speckled padloper tortoise at 8 centimeters (3.1 inches) to the leatherback sea turtle at 200 centimeters (over 6 feet). The term “turtle” is sometimes used to describe only those species of Testudines that live in the sea, with the terms “tortoise” and “terrapin” used to refer to species that live on land and in fresh water, respectively.
Summary
The amniotes are distinguished from amphibians by the presence of a terrestrially adapted egg protected by amniotic membranes. The amniotes include reptiles, birds, and mammals. The early amniotes diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids (mammals) and sauropsids. Sauropsids can be further divided into anapsids (turtles) and diapsids (birds and reptiles). Reptiles are tetrapods either having four limbs or descending from such. Limbless reptiles (snakes) are classified as tetrapods, as they are descended from four-limbed organisms. One of the key adaptations that permitted reptiles to live on land was the development of scaly skin containing the protein keratin, which prevented water loss from the skin. Reptilia includes four living clades: Crocodilia (crocodiles and alligators), Sphenodontia (tuataras), Squamata (lizards and snakes), and Testudines (turtles).
Art Connections
Figure \(1\): Which of the following statements about the parts of an egg are false?
1. The allantois stores nitrogenous waste and facilitates respiration.
2. The chorion facilitates gas exchange.
3. The yolk provides food for the growing embryo.
4. The amniotic cavity is filled with albumen.
Answer
D
Figure \(3\): Members of the order Testudines have an anapsid-like skull with one opening. However, molecular studies indicate that turtles descended from a diapsid ancestor. Why might this be the case?
Answer
The ancestor of modern Testudines may at one time have had a second opening in the skull, but over time this might have been lost.
Glossary
amniote
animal that produces a terrestrially adapted egg protected by amniotic membranes
allantois
membrane of the egg that stores nitrogenous wastes produced by the embryo; also facilitates respiration
amnion
membrane of the egg that protects the embryo from mechanical shock and prevents dehydration
anapsid
animal having no temporal fenestrae in the cranium
archosaur
modern crocodilian or bird, or an extinct pterosaur or dinosaur
brumation
period of much reduced metabolism and torpor that occurs in any ectotherm in cold weather
Casineria
one of the oldest known amniotes; had both amphibian and reptilian characteristics
chorion
membrane of the egg that surrounds the embryo and yolk sac
Crocodilia
crocodiles and alligators
diapsid
animal having two temporal fenestrae in the cranium
Hylonomus
one of the earliest reptiles
lepidosaur
modern lizards, snakes, and tuataras
sauropsid
reptile or bird
Sphenodontia
clade of tuataras
Squamata
clade of lizards and snakes
synapsid
mammal having one temporal fenestra
temporal fenestra
non-orbital opening in the skull that may allow muscles to expand and lengthen
Testudines
order of turtles | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/34%3A_Deuterostomes/34.07%3A_Reptiles.txt |
Skills to Develop
• Describe the evolutionary history of birds
• Describe the derived characteristics in birds that facilitate flight
The most obvious characteristic that sets birds apart from other modern vertebrates is the presence of feathers, which are modified scales. While vertebrates like bats fly without feathers, birds rely on feathers and wings, along with other modifications of body structure and physiology, for flight.
Characteristics of Birds
Birds are endothermic, and because they fly, they require large amounts of energy, necessitating a high metabolic rate. Like mammals, which are also endothermic, birds have an insulating covering that keeps heat in the body: feathers. Specialized feathers called down feathers are especially insulating, trapping air in spaces between each feather to decrease the rate of heat loss. Certain parts of a bird’s body are covered in down feathers, and the base of other feathers have a downy portion, whereas newly hatched birds are covered in down.
Feathers not only act as insulation but also allow for flight, enabling the lift and thrust necessary to become airborne. The feathers on a wing are flexible, so the collective feathers move and separate as air moves through them, reducing the drag on the wing. Flight feathers are asymmetrical, which affects airflow over them and provides some of the lifting and thrusting force required for flight (Figure \(1\)). Two types of flight feathers are found on the wings, primary feathers and secondary feathers. Primary feathers are located at the tip of the wing and provide thrust. Secondary feathers are located closer to the body, attach to the forearm portion of the wing and provide lift. Contour feathers are the feathers found on the body, and they help reduce drag produced by wind resistance during flight. They create a smooth, aerodynamic surface so that air moves smoothly over the bird’s body, allowing for efficient flight.
Flapping of the entire wing occurs primarily through the actions of the chest muscles, the pectoralis and the supracoracoideus. These muscles are highly developed in birds and account for a higher percentage of body mass than in most mammals. These attach to a blade-shaped keel, like that of a boat, located on the sternum. The sternum of birds is larger than that of other vertebrates, which accommodates the large muscles required to generate enough upward force to generate lift with the flapping of the wings. Another skeletal modification found in most birds is the fusion of the two clavicles (collarbones), forming the furcula or wishbone. The furcula is flexible enough to bend and provide support to the shoulder girdle during flapping.
An important requirement of flight is a low body weight. As body weight increases, the muscle output required for flying increases. The largest living bird is the ostrich, and while it is much smaller than the largest mammals, it is flightless. For birds that do fly, reduction in body weight makes flight easier. Several modifications are found in birds to reduce body weight, including pneumatization of bones. Pneumatic bones are bones that are hollow, rather than filled with tissue (Figure \(2\)). They contain air spaces that are sometimes connected to air sacs, and they have struts of bone to provide structural reinforcement. Pneumatic bones are not found in all birds, and they are more extensive in large birds than in small birds. Not all bones of the skeleton are pneumatic, although the skulls of almost all birds are.
Other modifications that reduce weight include the lack of a urinary bladder. Birds possess a cloaca, a structure that allows water to be reabsorbed from waste back into the bloodstream. Uric acid is not expelled as a liquid but is concentrated into urate salts, which are expelled along with fecal matter. In this way, water is not held in the urinary bladder, which would increase body weight. Most bird species only possess one ovary rather than two, further reducing body mass.
The air sacs that extend into bones to form pneumatic bones also join with the lungs and function in respiration. Unlike mammalian lungs in which air flows in two directions, as it is breathed in and out, airflow through bird lungs travels in one direction (Figure \(3\)). Air sacs allow for this unidirectional airflow, which also creates a cross-current exchange system with the blood. In a cross-current or counter-current system, the air flows in one direction and the blood flows in the opposite direction, creating a very efficient means of gas exchange.
Evolution of Birds
The evolutionary history of birds is still somewhat unclear. Due to the fragility of bird bones, they do not fossilize as well as other vertebrates. Birds are diapsids, meaning they have two fenestrations or openings in their skulls. Birds belong to a group of diapsids called the archosaurs, which also includes crocodiles and dinosaurs. It is commonly accepted that birds evolved from dinosaurs.
Dinosaurs (including birds) are further subdivided into two groups, the Saurischia (“lizard like”) and the Ornithischia (“bird like”). Despite the names of these groups, it was not the bird-like dinosaurs that gave rise to modern birds. Rather, Saurischia diverged into two groups: One included the long-necked herbivorous dinosaurs, such as Apatosaurus. The second group, bipedal predators called theropods, includes birds. This course of evolution is suggested by similarities between theropod fossils and birds, specifically in the structure of the hip and wrist bones, as well as the presence of the wishbone, formed by the fusing of the clavicles.
One important fossil of an animal intermediate to dinosaurs and birds is Archaeopteryx, which is from the Jurassic period (Figure \(4\)). Archaeopteryx is important in establishing the relationship between birds and dinosaurs, because it is an intermediate fossil, meaning it has characteristics of both dinosaurs and birds. Some scientists propose classifying it as a bird, but others prefer to classify it as a dinosaur. The fossilized skeleton of Archaeopteryx looks like that of a dinosaur, and it had teeth whereas birds do not, but it also had feathers modified for flight, a trait associated only with birds among modern animals. Fossils of older feathered dinosaurs exist, but the feathers do not have the characteristics of flight feathers.
It is still unclear exactly how flight evolved in birds. Two main theories exist, the arboreal (“tree”) hypothesis and the terrestrial (“land”) hypothesis. The arboreal hypothesis posits that tree-dwelling precursors to modern birds jumped from branch to branch using their feathers for gliding before becoming fully capable of flapping flight. In contrast to this, the terrestrial hypothesis holds that running was the stimulus for flight, as wings could be used to improve running and then became used for flapping flight. Like the question of how flight evolved, the question of how endothermy evolved in birds still is unanswered. Feathers provide insulation, but this is only beneficial if body heat is being produced internally. Similarly, internal heat production is only viable if insulation is present to retain that heat. It has been suggested that one or the other—feathers or endothermy—evolved in response to some other selective pressure.
During the Cretaceous period, a group known as the Enantiornithes was the dominant bird type (Figure \(5\)). Enantiornithes means “opposite birds,” which refers to the fact that certain bones of the feet are joined differently than the way the bones are joined in modern birds. These birds formed an evolutionary line separate from modern birds, and they did not survive past the Cretaceous. Along with the Enantiornithes, Ornithurae birds (the evolutionary line that includes modern birds) were also present in the Cretaceous. After the extinction of Enantiornithes, modern birds became the dominant bird, with a large radiation occurring during the Cenozoic Era. Referred to as Neornithes (“new birds”), modern birds are now classified into two groups, the Paleognathae (“old jaw”) or ratites, a group of flightless birds including ostriches, emus, rheas, and kiwis, and the Neognathae (“new jaw”), which includes all other birds.
Career Connection: Veterinarian
Veterinarians treat diseases, disorders, and injuries in animals, primarily vertebrates. They treat pets, livestock, and animals in zoos and laboratories. Veterinarians usually treat dogs and cats, but also treat birds, reptiles, rabbits, and other animals that are kept as pets. Veterinarians that work with farms and ranches treat pigs, goats, cows, sheep, and horses.
Veterinarians are required to complete a degree in veterinary medicine, which includes taking courses in animal physiology, anatomy, microbiology, and pathology, among many other courses. The physiology and biochemistry of different vertebrate species differ greatly.
Veterinarians are also trained to perform surgery on many different vertebrate species, which requires an understanding of the vastly different anatomies of various species. For example, the stomach of ruminants like cows has four compartments versus one compartment for non-ruminants. Birds also have unique anatomical adaptations that allow for flight.
Some veterinarians conduct research in academic settings, broadening our knowledge of animals and medical science. One area of research involves understanding the transmission of animal diseases to humans, called zoonotic diseases. For example, one area of great concern is the transmission of the avian flu virus to humans. One type of avian flu virus, H5N1, is a highly pathogenic strain that has been spreading in birds in Asia, Europe, Africa, and the Middle East. Although the virus does not cross over easily to humans, there have been cases of bird-to-human transmission. More research is needed to understand how this virus can cross the species barrier and how its spread can be prevented.
Summary
Birds are endothermic, meaning they produce their own body heat and regulate their internal temperature independently of the external temperature. Feathers not only act as insulation but also allow for flight, providing lift with secondary feathers and thrust with primary feathers. Pneumatic bones are bones that are hollow rather than filled with tissue, containing air spaces that are sometimes connected to air sacs. Airflow through bird lungs travels in one direction, creating a cross-current exchange with the blood. Birds are diapsids and belong to a group called the archosaurs. Birds are thought to have evolved from theropod dinosaurs. The oldest known fossil of a bird is that of Archaeopteryx, which is from the Jurassic period. Modern birds are now classified into two groups, Paleognathae and Neognathae.
Glossary
Archaeopteryx
transition species from dinosaur to bird from the Jurassic period
contour feather
feather that creates an aerodynamic surface for efficient flight
down feather
feather specialized for insulation
Enantiornithes
dominant bird group during the Cretaceous period
flight feather
feather specialized for flight
furcula
wishbone formed by the fusing of the clavicles
Neognathae
birds other than the Paleognathae
Neornithes
modern birds
Paleognathae
ratites; flightless birds, including ostriches and emus
pneumatic bone
air-filled bone
primary feather
feather located at the tip of the wing that provides thrust
secondary feather
feather located at the base of the wing that provides lift
theropod
dinosaur group ancestral to birds | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/34%3A_Deuterostomes/34.08%3A_Birds.txt |
Skills to Develop
• Name and describe the distinguishing features of the three main groups of mammals
• Describe the proposed line of descent that produced mammals
• List some derived features that may have arisen in response to mammals’ need for constant, high-level metabolism
Mammals are vertebrates that possess hair and mammary glands. Several other characteristics are distinctive to mammals, including certain features of the jaw, skeleton, integument, and internal anatomy. Modern mammals belong to three clades: monotremes, marsupials, and eutherians (or placental mammals).
Characteristics of Mammals
The presence of hair is one of the most obvious signs of a mammal. Although it is not very extensive on certain species, such as whales, hair has many important functions for mammals. Mammals are endothermic, and hair provides insulation to retain heat generated by metabolic work. Hair traps a layer of air close to the body, retaining heat. Along with insulation, hair can serve as a sensory mechanism via specialized hairs called vibrissae, better known as whiskers. These attach to nerves that transmit information about sensation, which is particularly useful to nocturnal or burrowing mammals. Hair can also provide protective coloration or be part of social signaling, such as when an animal’s hair stands “on end.”
Mammalian integument, or skin, includes secretory glands with various functions. Sebaceous glands produce a lipid mixture called sebum that is secreted onto the hair and skin for water resistance and lubrication. Sebaceous glands are located over most of the body. Eccrine glands produce sweat, or perspiration, which is mainly composed of water. In most mammals, eccrine glands are limited to certain areas of the body, and some mammals do not possess them at all. However, in primates, especially humans, sweat figures prominently in thermoregulation, regulating the body through evaporative cooling. Sweat glands are located over most of the body surface in primates. Apocrine glands, or scent glands, secrete substances that are used for chemical communication, such as in skunks. Mammary glands produce milk that is used to feed newborns. While male monotremes and eutherians possess mammary glands, male marsupials do not. Mammary glands likely are modified sebaceous or eccrine glands, but their evolutionary origin is not entirely clear.
The skeletal system of mammals possesses many unique features. The lower jaw of mammals consists of only one bone, the dentary. The jaws of other vertebrates are composed of more than one bone. In mammals, the dentary bone joins the skull at the squamosal bone, while in other vertebrates, the quadrate bone of the jaw joins with the articular bone of the skull. These bones are present in mammals, but they have been modified to function in hearing and form bones in the middle ear (Figure \(1\)). Other vertebrates possess only one middle ear bone, the stapes. Mammals have three: the malleus, incus, and stapes. The malleus originated from the articular bone, whereas the incus originated from the quadrate bone. This arrangement of jaw and ear bones aids in distinguishing fossil mammals from fossils of other synapsids.
The adductor muscle that closes the jaw is composed of two muscles in mammals: the temporalis and the masseter. These allow side-to-side movement of the jaw, making chewing possible, which is unique to mammals. Most mammals have heterodont teeth, meaning that they have different types and shapes of teeth rather than just one type and shape of tooth. Most mammals are diphyodonts, meaning that they have two sets of teeth in their lifetime: deciduous or “baby” teeth, and permanent teeth. Other vertebrates are polyphyodonts, that is, their teeth are replaced throughout their entire life.
Mammals, like birds, possess a four-chambered heart. Mammals also have a specialized group of cardiac fibers located in the walls of their right atrium called the sinoatrial node, or pacemaker, which determines the rate at which the heart beats. Mammalian erythrocytes (red blood cells) do not have nuclei, whereas the erythrocytes of other vertebrates are nucleated.
The kidneys of mammals have a portion of the nephron called the loop of Henle or nephritic loop, which allows mammals to produce urine with a high concentration of solutes, higher than that of the blood. Mammals lack a renal portal system, which is a system of veins that moves blood from the hind or lower limbs and region of the tail to the kidneys. Renal portal systems are present in all other vertebrates except jawless fishes. A urinary bladder is present in all mammals.
Mammalian brains have certain characteristics that differ from other vertebrates. In some, but not all mammals, the cerebral cortex, the outermost part of the cerebrum, is highly folded, allowing for a greater surface area than is possible with a smooth cortex. The optic lobes, located in the midbrain, are divided into two parts in mammals, whereas other vertebrates possess a single, undivided lobe. Eutherian mammals also possess a specialized structure that links the two cerebral hemispheres, called the corpus callosum.
Evolution of Mammals
Mammals are synapsids, meaning they have a single opening in the skull. They are the only living synapsids, as earlier forms became extinct by the Jurassic period. The early non-mammalian synapsids can be divided into two groups, the pelycosaurs and the therapsids. Within the therapsids, a group called the cynodonts are thought to be the ancestors of mammals (Figure \(2\)).
A key characteristic of synapsids is endothermy, rather than the ectothermy seen in most other vertebrates. The increased metabolic rate required to internally modify body temperature went hand in hand with changes to certain skeletal structures. The later synapsids, which had more evolved characteristics unique to mammals, possess cheeks for holding food and heterodont teeth, which are specialized for chewing, mechanically breaking down food to speed digestion and releasing the energy needed to produce heat. Chewing also requires the ability to chew and breathe at the same time, which is facilitated by the presence of a secondary palate. A secondary palate separates the area of the mouth where chewing occurs from the area above where respiration occurs, allowing breathing to proceed uninterrupted during chewing. A secondary palate is not found in pelycosaurs but is present in cynodonts and mammals. The jawbone also shows changes from early synapsids to later ones. The zygomatic arch, or cheekbone, is present in mammals and advanced therapsids such as cynodonts, but is not present in pelycosaurs. The presence of the zygomatic arch suggests the presence of the masseter muscle, which closes the jaw and functions in chewing.
In the appendicular skeleton, the shoulder girdle of therian mammals is modified from that of other vertebrates in that it does not possess a procoracoid bone or an interclavicle, and the scapula is the dominant bone.
Mammals evolved from therapsids in the late Triassic period, as the earliest known mammal fossils are from the early Jurassic period, some 205 million years ago. Early mammals were small, about the size of a small rodent. Mammals first began to diversify in the Mesozoic Era, from the Jurassic to the Cretaceous periods, although most of these mammals were extinct by the end of the Mesozoic. During the Cretaceous period, another radiation of mammals began and continued through the Cenozoic Era, about 65 million years ago.
Living Mammals
The eutherians, or placental mammals, and the marsupials together comprise the clade of therian mammals. Monotremes, or metatherians, form their sister clade.
There are three living species of monotremes: the platypus and two species of echidnas, or spiny anteaters. The leathery-beaked platypus belongs to the family Ornithorhynchidae (“bird beak”), whereas echidnas belong to the family Tachyglossidae (“sticky tongue”) (Figure \(3\)). The platypus and one species of echidna are found in Australia, and the other species of echidna is found in New Guinea. Monotremes are unique among mammals as they lay eggs, rather than giving birth to live young. The shells of their eggs are not like the hard shells of birds, but are a leathery shell, similar to the shells of reptile eggs. Monotremes have no teeth.
Marsupials are found primarily in Australia, though the opossum is found in North America. Australian marsupials include the kangaroo, koala, bandicoot, Tasmanian devil (Figure \(4\)), and several other species. Most species of marsupials possess a pouch in which the very premature young reside after birth, receiving milk and continuing to develop. Marsupials differ from eutherians in that there is a less complex placental connection: The young are born at an extremely early age and latch onto the nipple within the pouch.
Eutherians are the most widespread of the mammals, occurring throughout the world. There are 18 to 20 orders of placental mammals. Some examples are Insectivora, the insect eaters; Edentata, the toothless anteaters; Rodentia, the rodents; Cetacea, the aquatic mammals including whales; Carnivora, carnivorous mammals including dogs, cats, and bears; and Primates, which includes humans. Eutherian mammals are sometimes called placental mammals because all species possess a complex placenta that connects a fetus to the mother, allowing for gas, fluid, and nutrient exchange. While other mammals possess a less complex placenta or briefly have a placenta, all eutherians possess a complex placenta during gestation.
Summary
Mammals in general are vertebrates that possess hair and mammary glands. The mammalian integument includes various secretory glands, including sebaceous glands, eccrine glands, apocrine glands, and mammary glands. Mammals are synapsids, meaning that they have a single opening in the skull. A key characteristic of synapsids is endothermy rather than the ectothermy seen in other vertebrates. Mammals probably evolved from therapsids in the late Triassic period, as the earliest known mammal fossils are from the early Jurassic period. There are three groups of mammals living today: monotremes, marsupials, and eutherians. Monotremes are unique among mammals as they lay eggs, rather than giving birth to young. Eutherian mammals are sometimes called placental mammals, because all species possess a complex placenta that connects a fetus to the mother, allowing for gas, fluid, and nutrient exchange.
Glossary
apocrine gland
scent gland that secretes substances that are used for chemical communication
dentary
single bone that comprises the lower jaw of mammals
diphyodont
refers to the possession of two sets of teeth in a lifetime
eccrine gland
sweat gland
eutherian mammal
mammal that possesses a complex placenta, which connects a fetus to the mother; sometimes called placental mammals
heterodont tooth
different types of teeth that are modified for different purposes
mammal
one of the groups of endothermic vertebrates that possesses hair and mammary glands
mammary gland
in female mammals, a gland that produces milk for newborns
marsupial
one of the groups of mammals that includes the kangaroo, koala, bandicoot, Tasmanian devil, and several other species; young develop within a pouch
monotreme
egg-laying mammal
Ornithorhynchidae
clade that includes the duck-billed platypus
sebaceous gland
in mammals, a skin gland that produce a lipid mixture called sebum
Tachyglossidae
clade that includes the echidna or spiny anteater | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/34%3A_Deuterostomes/34.09%3A_Mammals.txt |
Skills to Develop
• Describe the derived features that distinguish primates from other animals
• Explain why scientists are having difficulty determining the true lines of descent in hominids
Order Primates of class Mammalia includes lemurs, tarsiers, monkeys, apes, and humans. Non-human primates live primarily in the tropical or subtropical regions of South America, Africa, and Asia. They range in size from the mouse lemur at 30 grams (1 ounce) to the mountain gorilla at 200 kilograms (441 pounds). The characteristics and evolution of primates is of particular interest to us as it allows us to understand the evolution of our own species.
Characteristics of Primates
All primate species possess adaptations for climbing trees, as they all descended from tree-dwellers. This arboreal heritage of primates has resulted in hands and feet that are adapted for brachiation, or climbing and swinging through trees. These adaptations include, but are not limited to: 1) a rotating shoulder joint, 2) a big toe that is widely separated from the other toes and thumbs, which are widely separated from fingers (except humans), which allow for gripping branches, 3) stereoscopic vision, two overlapping fields of vision from the eyes, which allows for the perception of depth and gauging distance. Other characteristics of primates are brains that are larger than those of most other mammals, claws that have been modified into flattened nails, typically only one offspring per pregnancy, and a trend toward holding the body upright.
Order Primates is divided into two groups: prosimians and anthropoids. Prosimians include the bush babies of Africa, the lemurs of Madagascar, and the lorises, pottos, and tarsiers of Southeast Asia. Anthropoids include monkeys, apes, and humans. In general, prosimians tend to be nocturnal (in contrast to diurnal anthropoids) and exhibit a smaller size and smaller brain than anthropoids.
Evolution of Primates
The first primate-like mammals are referred to as proto-primates. They were roughly similar to squirrels and tree shrews in size and appearance. The existing fossil evidence (mostly from North Africa) is very fragmented. These proto-primates remain largely mysterious creatures until more fossil evidence becomes available. The oldest known primate-like mammals with a relatively robust fossil record is Plesiadapis (although some researchers do not agree that Plesiadapis was a proto-primate). Fossils of this primate have been dated to approximately 55 million years ago. Plesiadapiforms were proto-primates that had some features of the teeth and skeleton in common with true primates. They were found in North America and Europe in the Cenozoic and went extinct by the end of the Eocene.
The first true primates were found in North America, Europe, Asia, and Africa in the Eocene Epoch. These early primates resembled present-day prosimians such as lemurs. Evolutionary changes continued in these early primates, with larger brains and eyes, and smaller muzzles being the trend. By the end of the Eocene Epoch, many of the early prosimian species went extinct due either to cooler temperatures or competition from the first monkeys.
Anthropoid monkeys evolved from prosimians during the Oligocene Epoch. By 40 million years ago, evidence indicates that monkeys were present in the New World (South America) and the Old World (Africa and Asia). New World monkeys are also called Platyrrhini—a reference to their broad noses (Figure \(1\)). Old World monkeys are called Catarrhini—a reference to their narrow noses. There is still quite a bit of uncertainty about the origins of the New World monkeys. At the time the platyrrhines arose, the continents of South American and Africa had drifted apart. Therefore, it is thought that monkeys arose in the Old World and reached the New World either by drifting on log rafts or by crossing land bridges. Due to this reproductive isolation, New World monkeys and Old World monkeys underwent separate adaptive radiations over millions of years. The New World monkeys are all arboreal, whereas Old World monkeys include arboreal and ground-dwelling species.
Apes evolved from the catarrhines in Africa midway through the Cenozoic, approximately 25 million years ago. Apes are generally larger than monkeys and they do not possess a tail. All apes are capable of moving through trees, although many species spend most their time on the ground. Apes are more intelligent than monkeys, and they have relatively larger brains proportionate to body size. The apes are divided into two groups. The lesser apes comprise the family Hylobatidae, including gibbons and siamangs. The great apes include the genera Pan (chimpanzees and bonobos) (Figure \(2\)a), Gorilla (gorillas), Pongo (orangutans), and Homo (humans) (Figure \(2\)b). The very arboreal gibbons are smaller than the great apes; they have low sexual dimorphism (that is, the genders are not markedly different in size); and they have relatively longer arms used for swinging through trees.
Human Evolution
The family Hominidae of order Primates includes the hominoids: the great apes (Figure \(3\)). Evidence from the fossil record and from a comparison of human and chimpanzee DNA suggests that humans and chimpanzees diverged from a common hominoid ancestor approximately 6 million years ago. Several species evolved from the evolutionary branch that includes humans, although our species is the only surviving member. The term hominin is used to refer to those species that evolved after this split of the primate line, thereby designating species that are more closely related to humans than to chimpanzees. Hominins were predominantly bipedal and include those groups that likely gave rise to our species—including Australopithecus, Homo habilis, and Homo erectus—and those non-ancestral groups that can be considered “cousins” of modern humans, such as Neanderthals. Determining the true lines of descent in hominins is difficult. In years past, when relatively few hominin fossils had been recovered, some scientists believed that considering them in order, from oldest to youngest, would demonstrate the course of evolution from early hominins to modern humans. In the past several years, however, many new fossils have been found, and it is clear that there was often more than one species alive at any one time and that many of the fossils found (and species named) represent hominin species that died out and are not ancestral to modern humans.
Very Early Hominins
Three species of very early hominids have made news in the past few years. The oldest of these, Sahelanthropus tchadensis, has been dated to nearly 7 million years ago. There is a single specimen of this genus, a skull that was a surface find in Chad. The fossil, informally called “Toumai,” is a mosaic of primitive and evolved characteristics, and it is unclear how this fossil fits with the picture given by molecular data, namely that the line leading to modern humans and modern chimpanzees apparently bifurcated about 6 million years ago. It is not thought at this time that this species was an ancestor of modern humans.
A second, younger species, Orrorin tugenensis, is also a relatively recent discovery, found in 2000. There are several specimens of Orrorin. It is not known whether Orrorin was a human ancestor, but this possibility has not been ruled out. Some features of Orrorin are more similar to those of modern humans than are the australopiths, although Orrorin is much older.
A third genus, Ardipithecus, was discovered in the 1990s, and the scientists who discovered the first fossil found that some other scientists did not believe the organism to be a biped (thus, it would not be considered a hominid). In the intervening years, several more specimens of Ardipithecus, classified as two different species, demonstrated that the organism was bipedal. Again, the status of this genus as a human ancestor is uncertain.
Early Hominins: Genus Australopithecus
Australopithecus (“southern ape”) is a genus of hominin that evolved in eastern Africa approximately 4 million years ago and went extinct about 2 million years ago. This genus is of particular interest to us as it is thought that our genus, genus Homo, evolved from Australopithecus about 2 million years ago (after likely passing through some transitional states). Australopithecus had a number of characteristics that were more similar to the great apes than to modern humans. For example, sexual dimorphism was more exaggerated than in modern humans. Males were up to 50 percent larger than females, a ratio that is similar to that seen in modern gorillas and orangutans. In contrast, modern human males are approximately 15 to 20 percent larger than females. The brain size of Australopithecus relative to its body mass was also smaller than modern humans and more similar to that seen in the great apes. A key feature that Australopithecus had in common with modern humans was bipedalism, although it is likely that Australopithecus also spent time in trees. Hominin footprints, similar to those of modern humans, were found in Laetoli, Tanzania and dated to 3.6 million years ago. They showed that hominins at the time of Australopithecus were walking upright.
There were a number of Australopithecus species, which are often referred to as australopiths. Australopithecus anamensis lived about 4.2 million years ago. More is known about another early species, Australopithecus afarensis, which lived between 3.9 and 2.9 million years ago. This species demonstrates a trend in human evolution: the reduction of the dentition and jaw in size. A. afarensis (Figure \(4\)) had smaller canines and molars compared to apes, but these were larger than those of modern humans. Its brain size was 380–450 cubic centimeters, approximately the size of a modern chimpanzee brain. It also had prognathic jaws, which is a relatively longer jaw than that of modern humans. In the mid-1970s, the fossil of an adult female A. afarensis was found in the Afar region of Ethiopia and dated to 3.24 million years ago (Figure \(5\)). The fossil, which is informally called “Lucy,” is significant because it was the most complete australopith fossil found, with 40 percent of the skeleton recovered.
Australopithecus africanus lived between 2 and 3 million years ago. It had a slender build and was bipedal, but had robust arm bones and, like other early hominids, may have spent significant time in trees. Its brain was larger than that of A. afarensis at 500 cubic centimeters, which is slightly less than one-third the size of modern human brains. Two other species, Australopithecus bahrelghazali and Australopithecus garhi, have been added to the roster of australopiths in recent years.
A Dead End: Genus Paranthropus
The australopiths had a relatively slender build and teeth that were suited for soft food. In the past several years, fossils of hominids of a different body type have been found and dated to approximately 2.5 million years ago. These hominids, of the genus Paranthropus, were relatively large and had large grinding teeth. Their molars showed heavy wear, suggesting that they had a coarse and fibrous vegetarian diet as opposed to the partially carnivorous diet of the australopiths. Paranthropus includes Paranthropus robustus of South Africa, and Paranthropus aethiopicus and Paranthropus boisei of East Africa. The hominids in this genus went extinct more than 1 million years ago and are not thought to be ancestral to modern humans, but rather members of an evolutionary branch on the hominin tree that left no descendants.
Early Hominins: Genus Homo
The human genus, Homo, first appeared between 2.5 and 3 million years ago. For many years, fossils of a species called H. habilis were the oldest examples in the genus Homo, but in 2010, a new species called Homo gautengensis was discovered and may be older. Compared to A. africanus, H. habilis had a number of features more similar to modern humans. H. habilis had a jaw that was less prognathic than the australopiths and a larger brain, at 600–750 cubic centimeters. However, H. habilis retained some features of older hominin species, such as long arms. The name H. habilis means “handy man,” which is a reference to the stone tools that have been found with its remains.
Link to Learning
Visit this site for a video about Smithsonian paleontologist Briana Pobiner explaining the link between hominin eating of meat and evolutionary trends.
H. erectus appeared approximately 1.8 million years ago (Figure \(6\)). It is believed to have originated in East Africa and was the first hominin species to migrate out of Africa. Fossils of H. erectus have been found in India, China, Java, and Europe, and were known in the past as “Java Man” or “Peking Man.” H. erectus had a number of features that were more similar to modern humans than those of H. habilis. H. erectus was larger in size than earlier hominins, reaching heights up to 1.85 meters and weighing up to 65 kilograms, which are sizes similar to those of modern humans. Its degree of sexual dimorphism was less than earlier species, with males being 20 to 30 percent larger than females, which is close to the size difference seen in our species. H. erectus had a larger brain than earlier species at 775–1,100 cubic centimeters, which compares to the 1,130–1,260 cubic centimeters seen in modern human brains. H. erectus also had a nose with downward-facing nostrils similar to modern humans, rather than the forward facing nostrils found in other primates. Longer, downward-facing nostrils allow for the warming of cold air before it enters the lungs and may have been an adaptation to colder climates. Artifacts found with fossils of H. erectus suggest that it was the first hominin to use fire, hunt, and have a home base. H. erectus is generally thought to have lived until about 50,000 years ago.
Humans: Homo sapiens
A number of species, sometimes called archaic Homo sapiens, apparently evolved from H. erectus starting about 500,000 years ago. These species include Homo heidelbergensis, Homo rhodesiensis, and Homo neanderthalensis. These archaic H. sapiens had a brain size similar to that of modern humans, averaging 1,200–1,400 cubic centimeters. They differed from modern humans by having a thick skull, a prominent brow ridge, and a receding chin. Some of these species survived until 30,000–10,000 years ago, overlapping with modern humans (Figure \(7\)).
There is considerable debate about the origins of anatomically modern humans or Homo sapiens sapiens. As discussed earlier, H. erectus migrated out of Africa and into Asia and Europe in the first major wave of migration about 1.5 million years ago. It is thought that modern humans arose in Africa from H. erectus and migrated out of Africa about 100,000 years ago in a second major migration wave. Then, modern humans replaced H. erectus species that had migrated into Asia and Europe in the first wave.
This evolutionary timeline is supported by molecular evidence. One approach to studying the origins of modern humans is to examine mitochondrial DNA (mtDNA) from populations around the world. Because a fetus develops from an egg containing its mother’s mitochondria (which have their own, non-nuclear DNA), mtDNA is passed entirely through the maternal line. Mutations in mtDNA can now be used to estimate the timeline of genetic divergence. The resulting evidence suggests that all modern humans have mtDNA inherited from a common ancestor that lived in Africa about 160,000 years ago. Another approach to the molecular understanding of human evolution is to examine the Y chromosome, which is passed from father to son. This evidence suggests that all men today inherited a Y chromosome from a male that lived in Africa about 140,000 years ago.
Summary
All primate species possess adaptations for climbing trees, as they all probably descended from tree-dwellers, although not all species are arboreal. Other characteristics of primates are brains that are larger than those of other mammals, claws that have been modified into flattened nails, typically only one young per pregnancy, stereoscopic vision, and a trend toward holding the body upright. Primates are divided into two groups: prosimians and anthropoids. Monkeys evolved from prosimians during the Oligocene Epoch. Apes evolved from catarrhines in Africa during the Miocene Epoch. Apes are divided into the lesser apes and the greater apes. Hominins include those groups that gave rise to our species, such as Australopithecus and H. erectus, and those groups that can be considered “cousins” of humans, such as Neanderthals. Fossil evidence shows that hominins at the time of Australopithecus were walking upright, the first evidence of bipedal hominins. A number of species, sometimes called archaic H. sapiens, evolved from H. erectus approximately 500,000 years ago. There is considerable debate about the origins of anatomically modern humans or H. sapiens sapiens.
Glossary
anthropoid
monkeys, apes, and humans
Australopithecus
genus of hominins that evolved in eastern Africa approximately 4 million years ago
brachiation
movement through trees branches via suspension from the arms
Catarrhini
clade of Old World monkeys
Gorilla
genus of gorillas
hominin
species that are more closely related to humans than chimpanzees
hominoid
pertaining to great apes and humans
Homo
genus of humans
Homo sapiens sapiens
anatomically modern humans
Hylobatidae
family of gibbons
Pan
genus of chimpanzees and bonobos
Platyrrhini
clade of New World monkeys
Plesiadapis
oldest known primate-like mammal
Pongo
genus of orangutans
Primates
order of lemurs, tarsiers, monkeys, apes, and humans
prognathic jaw
long jaw
prosimian
division of primates that includes bush babies of Africa, lemurs of Madagascar, and lorises, pottos, and tarsiers of Southeast Asia
stereoscopic vision
two overlapping fields of vision from the eyes that produces depth perception | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/34%3A_Deuterostomes/34.10%3A_Evolution_of_Primates.txt |
Learning Objectives
• Differentiate among the types of plant tissues and organs
Plant Tissues
Plants are multicellular eukaryotes with tissue systems made of various cell types that carry out specific functions. Plant tissue systems fall into one of two general types: meristematic tissue and permanent (or non-meristematic) tissue. Cells of the meristematic tissue are found in meristems, which are plant regions of continuous cell division and growth. Meristematic tissue cells are either undifferentiated or incompletely differentiated; they continue to divide and contribute to the growth of the plant. In contrast, permanent tissue consists of plant cells that are no longer actively dividing.
Meristematic tissues consist of three types, based on their location in the plant. Apical meristems contain meristematic tissue located at the tips of stems and roots, which enable a plant to extend in length. Lateral meristems facilitate growth in thickness or girth in a maturing plant. Intercalary meristems occur only in monocots at the bases of leaf blades and at nodes (the areas where leaves attach to a stem). This tissue enables the monocot leaf blade to increase in length from the leaf base; for example, it allows lawn grass leaves to elongate even after repeated mowing.
Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. Such cells take on specific roles and lose their ability to divide further. They differentiate into three main types: dermal, vascular, and ground tissue. Dermal tissue covers and protects the plant. Vascular tissue transports water, minerals, and sugars to different parts of the plant. Ground tissue serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps to store water and sugars.
Plant tissues are either simple (composed of similar cell types) or complex (composed of different cell types). Dermal tissue, for example, is a simple tissue that covers the outer surface of the plant and controls gas exchange. Vascular tissue is an example of a complex tissue. It is made of two specialized conducting tissues: xylem and phloem. Xylem tissue transports water and nutrients from the roots to different parts of the plant. It includes three different cell types: vessel elements and tracheids (both of which conduct water) and xylem parenchyma. Phloem tissue, which transports organic compounds from the site of photosynthesis to other parts of the plant, consists of four different cell types: sieve cells (which conduct photosynthates), companion cells, phloem parenchyma, and phloem fibers. Unlike xylem-conducting cells, phloem-conducting cells are alive at maturity. The xylem and phloem always lie adjacent to each other. In stems, the xylem and the phloem form a structure called a vascular bundle; in roots, this is termed the vascular stele or vascular cylinder.
Plant Organ Systems
In plants, just as in animals, similar cells working together form a tissue. When different types of tissues work together to perform a unique function, they form an organ; organs working together form organ systems. Vascular plants have two distinct organ systems: a shoot system and a root system. The shoot system consists of two portions: the vegetative (non-reproductive) parts of the plant, such as the leaves and the stems; and the reproductive parts of the plant, which include flowers and fruits. The shoot system generally grows above ground, where it absorbs the light needed for photosynthesis. The root system, which supports the plants and absorbs water and minerals, is usually underground.
Key Points
• There are two types of plant tissues: meristematic tissue found in plant regions of continuous cell division and growth, and permanent (or non-meristematic) tissue consisting of cells that are no longer actively dividing.
• Meristems produce cells that differentiate into three secondary tissue types: dermal tissue which covers and protects the plant, vascular tissue which transports water, minerals, and sugars and ground tissue which serves as a site for photosynthesis, supports vascular tissue, and stores nutrients.
• Vascular tissue is made of xylem tissue which transports water and nutrients from the roots to different parts of the plant and phloem tissue which transports organic compounds from the site of photosynthesis to other parts of the plant.
• The xylem and phloem always lie next to each other forming a structure called a vascular bundle in stems and a vascular stele or vascular cylinder in roots.
• Parts of the shoot system include the vegetative parts, such as the leaves and the stems, and the reproductive parts, such as the flowers and fruits.
Key Terms
• meristem: the plant tissue composed of totipotent cells that allows plant growth
• parenchyma: the ground tissue making up most of the non-woody parts of a plant
• xylem: a vascular tissue in land plants primarily responsible for the distribution of water and minerals taken up by the roots; also the primary component of wood
• phloem: a vascular tissue in land plants primarily responsible for the distribution of sugars and nutrients manufactured in the shoot
• tracheid: elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/35%3A_Plant_Form/35.01%3A_Organization_of_the_Plant_Body-_An_Overview/35.1A%3A_Plant_Tissues_and_Organ_Systems.txt |
Meristematic
The main function of meristematic tissue is mitosis. The cells are small, thin-walled, with no central vacuole and no specialized features.
Meristematic tissue is located in
• the apical meristems at the growing points of roots and stems.
• the secondary meristems (lateral buds) at the nodes of stems (where branching occurs), and in some plants,
• meristematic tissue, called the cambium, that is found within mature stems and roots.
The cells produced in the meristems soon become differentiated into one or another of several types.
Protective
Protective tissue covers the surface of leaves and the living cells of roots and stems. Its cells are flattened with their top and bottom surfaces parallel. The upper and lower epidermis of the leaf are examples of protective tissue.
Parenchyma
The cells of parenchyma are large, thin-walled, and usually have a large central vacuole. They are often partially separated from each other and are usually stuffed with plastids. In areas not exposed to light, colorless plastids predominate and food storage is the main function. The cells of the white potato are parenchyma cells. Where light is present, e.g., in leaves, chloroplasts predominate and photosynthesis is the main function.
Sclerenchyma
The walls of these cells are very thick and built up in a uniform layer around the entire margin of the cell. Often, the cell dies after its cell wall is fully formed. Sclerenchyma cells are usually found associated with other cells types and give them mechanical support.
Sclerenchyma is found in stems and also in leaf veins. Sclerenchyma also makes up the hard outer covering of seeds and nuts.
Collenchyma
Collenchyma cells have thick walls that are especially thick at their corners. These cells provide mechanical support for the plant. They are most often found in areas that are growing rapidly and need to be strengthened. The petiole ("stalk") of leaves is usually reinforced with collenchyma.
Xylem
Xylem conducts water and dissolved minerals from the roots to all the other parts of the plant.
In angiosperms, most of the water travels in the xylem vessels. These are thick-walled tubes that can extend vertically through several feet of xylem tissue. Their diameter may be as large as 0.7 mm. Their walls are thickened with secondary deposits of cellulose and are usually further strengthened by impregnation with lignin. The secondary walls of the xylem vessels are deposited in spirals and rings and are usually perforated by pits. Xylem vessels arise from individual cylindrical cells oriented end to end. At maturity the end walls of these cells dissolve away, and the cytoplasmic contents die. The result is the xylem vessel, a continuous nonliving duct.
Xylem also contains tracheids. These are individual cells tapered at each end so the tapered end of one cell overlaps that of the adjacent cell. Like xylem vessels, they have thick, lignified walls and, at maturity, no cytoplasm. Their walls are perforated so that water can flow from one tracheid to the next. The xylem of ferns and conifers contains only tracheids.
In woody plants, the older xylem ceases to participate in water transport and simply serves to give strength to the trunk. Wood is xylem. When counting the annual rings of a tree, one is counting rings of xylem.
Phloem
The main components of phloem are sieve elements and companion cells.
Sieve elements are so named because their end walls are perforated. This allows cytoplasmic connections between vertically-stacked cells. The result is a sieve tube that conducts the products of photosynthesis - sugars and amino acids - from the place where they are manufactured (a "source"), e.g., leaves, to the places ("sinks") where they are consumed or stored; such as
• roots
• growing tips of stems and leaves
• flowers
• fruits, tubers, corms, etc.
Sieve elements have no nucleus and only a sparse collection of other organelles. They depend on the adjacent companion cells for many functions. Companion cells move sugars, amino acids and a variety of macromolecules into and out of the sieve elements. In "source" tissue, such as a leaf, the companion cells use transmembrane proteins to take up - by active transport - sugars and other organic molecules from the cells manufacturing them. Water follows by osmosis. These materials then move into adjacent sieve elements through plasmodesmata. The pressure created by osmosis drives the flow of materials through the sieve tubes.
In "sink" tissue, the sugars and other organic molecules leave the sieve elements through plasmodesmata connecting the sieve elements to their companion cells and then pass on to the cells of their destination. Again, water follows by osmosis where it may leave the plant by transpiration or increase the volume of the cells or move into the xylem for recycling through the plant. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/35%3A_Plant_Form/35.02%3A_Plant_Tissues/35.2.01%3A_Plant_Tissues.txt |
Learning Objectives
• Describe the three zones of the root tip and summarize the role of each zone in root growth
Types of Root Systems
There are two main types of root systems. Dicots have a tap root system, while monocots have a fibrous root system, which is also known as an adventitious root system. A tap root system has a main root that grows down vertically, from which many smaller lateral roots arise. Dandelions are a common example; their tap roots usually break off when these weeds are pulled from the ground; they can regrow another shoot from the remaining root. A tap root system penetrates deep into the soil. In contrast, a fibrous root system is located closer to the soil surface where it forms a dense network of roots that also helps prevent soil erosion (lawn grasses are a good example, as are wheat, rice, and corn). Some plants have a combination of tap roots and fibrous roots. Plants that grow in dry areas often have deep root systems, whereas plants that grow in areas with abundant water are likely to have shallower root systems.
Zones of the Root Tip
Root growth begins with seed germination. When the plant embryo emerges from the seed, the radicle of the embryo forms the root system. The tip of the root is protected by the root cap, a structure exclusive to roots and unlike any other plant structure. The root cap is continuously replaced because it is easily damaged as the root pushes through soil. The root tip can be divided into three zones: a zone of cell division, a zone of elongation, and a zone of maturation. The zone of cell division is closest to the root tip and is made up of the actively-dividing cells of the root meristem, which contains the undifferentiated cells of the germinating plant. The zone of elongation is where the newly-formed cells increase in length, thereby lengthening the root. Beginning at the first root hair is the zone of cell maturation where the root cells differentiate into specialized cell types. All three zones are in approximately the first centimeter of the root tip.
Key Points
• Root tips ultimately develop into two main types of root systems: tap roots and fibrous roots.
• The growing root tip is protected by a root cap.
• Within the root tip, cells differentiate, actively divide, and increase in length, depending on in which zone the cells are located.
• Dividing cells make up the zone of cell division in a germinating plant.
• The newly-forming root increases in size in the zone of elongation.
• Differentiating cells make up the zone of cell maturation.
Key Terms
• radicle: the rudimentary shoot of a plant that supports the cotyledons in the seed and from which the root is developed downward; the root of the embryo
• meristem: the plant tissue composed of totipotent cells that allows plant growth
• germination: the beginning of vegetation or growth from a seed or spore
35.3B: Root Modifications
Learning Objectives
• Explain the reasons for root modifications
Plants have different root structures for specific purposes. There are many different types of specialized roots, but two of the more familiar types of roots include aerial roots and storage roots. Aerial roots grow above the ground, typically providing structural support. Storage roots (for example, taproots and tuberous roots) are modified for food storage.
Aerial roots are found in many different kinds of plants, offering varying functions depending on the location of the plant. Epiphytic roots are a type of aerial root that enable a plant to grow on another plant in a non-parasitic manner. The banyan tree begins as an epiphyte, germinating in the branches of a host tree. Aerial prop roots develop from the branches and eventually reach the ground, providing additional support. Over time, many roots will come together to form what appears to be a trunk. The epiphytic roots of orchids develop a spongy tissue to absorb moisture and nutrients from any organic material on their roots. In screwpine, a palm-like tree that grows in sandy tropical soils, aerial roots develop to provide additional support that help the tree remain upright in shifting sand and water conditions.
Storage roots, such as carrots, beets, and sweet potatoes, are examples of roots that are specially modified for storage of starch and water. They usually grow underground as protection from plant-eating animals. Some plants, however, such as leaf succulents and cacti, store energy in their leaves and stems, respectively, instead of in their roots.
Other examples of modified roots are aerating roots and haustorial roots. Aerating roots, which rise above the ground, especially above water, are commonly seen in mangrove forests that grow along salt water coastlines. Haustorial roots are often seen in parasitic plants such as mistletoe. Their roots allow the plants to absorb water and nutrients from other plants.
Key Points
• Storage roots, which include a large number of edible vegetables such as potatoes and carrots, are some of the most commonly-known types of modified roots.
• Aerial roots encompass a variety of shapes, yet function similarly as structural support for the plant.
• Parasitic plants have special haustorial roots that allow the plant to absorb nutrients from a host plant.
Key Terms
• succulent: having fleshy leaves or other tissues that store water
• epiphyte: a plant that grows on another, using it as a physical support but neither obtaining nutrients from it nor causing it any damage if also offering no benefit | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/35%3A_Plant_Form/35.03%3A_Roots-_Anchoring_and_Absorption_Structures/35.3A%3A__Types_of_Root_Systems_and_Zones_of_Growth.txt |
Learning Objectives
• Summarize the main function and basic structure of stems
Stems
Stems are a part of the shoot system of a plant. They may range in length from a few millimeters to hundreds of meters. They also vary in diameter, depending on the plant type. Stems are usually above ground, although the stems of some plants, such as the potato, also grow underground. Stems may be herbaceous (soft) or woody in nature. Their main function is to provide support to the plant, holding leaves, flowers, and buds; in some cases, stems also store food for the plant. A stem may be unbranched, like that of a palm tree, or it may be highly branched, like that of a magnolia tree. The stem of the plant connects the roots to the leaves, helping to transport absorbed water and minerals to different parts of the plant. The stem also helps to transport the products of photosynthesis (i.e., sugars) from the leaves to the rest of the plant.
Plant stems, whether above or below ground, are characterized by the presence of nodes and internodes. Nodes are points of attachment for leaves, aerial roots, and flowers. The stem region between two nodes is called an internode. The stalk that extends from the stem to the base of the leaf is the petiole. An axillary bud is usually found in the axil (the area between the base of a leaf and the stem) where it can give rise to a branch or a flower. The apex (tip) of the shoot contains the apical meristem within the apical bud.
Key Points
• Most stems are found above ground, but some of them grow underground.
• Stems can be either unbranched or highly branched; they may be herbaceous or woody.
• Stems connect the roots to the leaves, helping to transport water, minerals, and sugars to different parts of the plant.
• Plant stems always have nodes (points of attachments for leaves, roots, and flowers) and internodes (regions between nodes).
• The petiole is the stalk that extends from the stem to the base of the leaf.
• An axillary bud gives rise to a branch or a flower; it is usually found in the axil: the junction of the stem and petiole.
Key Terms
• node: points of attachment for leaves, aerial roots, and flowers
• internode: a section of stem between two stem nodes
• petiole: stalk that extends from the stem to the base of the leaf
• axillary bud: embryonic shoot that lies at the junction of the stem and petiole that gives rise to a branch or flower
35.4B: Stem Anatomy
Learning Objectives
• Summarize the roles of dermal tissue, vascular tissue, and ground tissue
Stem Anatomy
The stem and other plant organs are primarily made from three simple cell types: parenchyma, collenchyma, and sclerenchyma cells. Parenchyma cells are the most common plant cells. They are found in the stem, the root, the inside of the leaf, and the pulp of the fruit. Parenchyma cells are responsible for metabolic functions, such as photosynthesis. They also help repair and heal wounds. In addition, some parenchyma cells store starch.
Collenchyma cells are elongated cells with unevenly-thickened walls. They provide structural support, mainly to the stem and leaves. These cells are alive at maturity and are usually found below the epidermis. The “strings” of a celery stalk are an example of collenchyma cells.
Sclerenchyma cells also provide support to the plant, but unlike collenchyma cells, many of them are dead at maturity. There are two types of sclerenchyma cells: fibers and sclereids. Both types have secondary cell walls that are thickened with deposits of lignin, an organic compound that is a key component of wood. Fibers are long, slender cells; sclereids are smaller-sized. Sclereids give pears their gritty texture. Humans use sclerenchyma fibers to make linen and rope.
As with the rest of the plant, the stem has three tissue systems: dermal, vascular, and ground tissue. Each is distinguished by characteristic cell types that perform specific tasks necessary for the plant’s growth and survival.
Dermal Tissue
The dermal tissue of the stem consists primarily of epidermis: a single layer of cells covering and protecting the underlying tissue. Woody plants have a tough, waterproof outer layer of cork cells commonly known as bark, which further protects the plant from damage. Epidermal cells are the most-numerous and least-differentiated of the cells in the epidermis. The epidermis of a leaf also contains openings, known as stomata, through which the exchange of gases takes place. Two cells, known as guard cells, surround each leaf stoma, controlling its opening and closing and, thus, regulating the uptake of carbon dioxide and the release of oxygen and water vapor. Trichomes are hair-like structures on the epidermal surface. They help to reduce transpiration (the loss of water by aboveground plant parts), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores.
Vascular Tissue
The xylem and phloem that make up the vascular tissue of the stem are arranged in distinct strands called vascular bundles, which run up and down the length of the stem. Both are considered complex plant tissue because they are composed of more than one simple cell type that work in concert with each other. When the stem is viewed in cross section, the vascular bundles of dicot stems are arranged in a ring. In plants with stems that live for more than one year, the individual bundles grow together and produce the characteristic growth rings. In monocot stems, the vascular bundles are randomly scattered throughout the ground tissue.
Xylem tissue has three types of cells: xylem parenchyma, tracheids, and vessel elements. The latter two types conduct water and are dead at maturity. Tracheids are xylem cells with thick secondary cell walls that are lignified. Water moves from one tracheid to another through regions on the side walls known as pits where secondary walls are absent. Vessel elements are xylem cells with thinner walls; they are shorter than tracheids. Each vessel element is connected to the next by means of a perforation plate at the end walls of the element. Water moves through the perforation plates to travel up the plant.
Phloem tissue is composed of sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers. A series of sieve-tube cells (also called sieve-tube elements) are arranged end-to-end to create a long sieve tube, which transports organic substances such as sugars and amino acids. The sugars flow from one sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between two cells. Although still alive at maturity, the nucleus and other cell components of the sieve-tube cells have disintegrated. Companion cells are found alongside the sieve-tube cells, providing them with metabolic support. The companion cells contain more ribosomes and mitochondria than do the sieve-tube cells, which lack some cellular organelles.
Ground Tissue
Ground tissue is mostly made up of parenchyma cells, but may also contain collenchyma and sclerenchyma cells that help support the stem. The ground tissue towards the interior of the vascular tissue in a stem or root is known as pith, while the layer of tissue between the vascular tissue and the epidermis is known as the cortex.
Key Points
• The stem has three simple cell types: the parenchyma, collenchyma, and sclerenchyma cells that are responsible for metabolic functions, repairing and healing wounds, and storing starch.
• The stem is composed of three tissue systems that include the epidermis, vascular, and ground tissues, all of which are made from the simple cell types..
• The xylem and phloem carry water and nutrients up and down the length of the stem and are arranged in distinct strands called vascular bundles.
• The epidermis is a single layer of cells that makes up the dermal tissue covering the stem and protecting the underlying tissue. Woody plants have an extra layer of protection on top of the epidermis made of cork cells known as bark.
• The vascular tissue of the stem consists of the complex tissues xylem and phloem which carry water and nutrients up and down the length of the stem and are arranged in distinct strands called vascular bundles.
• Ground tissue helps support the stem and is called pith when it is located towards the middle of the stem and called the cortex when it is between the vascular tissue and the epidermis.
Key Terms
• collenchyma: a supporting ground tissue just under the surface of various leaf structures formed before vascular differentiation
• sclerenchyma: a mechanical, supportive ground tissue in plants consisting of aggregates of cells having thick, often mineralized walls
• sclereid: a reduced form of sclerenchyma cells with highly-thickened, lignified walls
• lignin: a complex, non-carbohydrate, aromatic polymer present in all wood
• stoma: a pore found in the leaf and stem epidermis used for gaseous exchange
• trichome: a hair- or scale-like extension of the epidermis of a plant
• xylem: a vascular tissue in land plants primarily responsible for the distribution of water and minerals taken up by the roots; also the primary component of wood
• phloem: a vascular tissue in land plants primarily responsible for the distribution of sugars and nutrients manufactured in the shoot
• tracheid: elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts
• pith: the soft spongy substance in the center of the stems of many plants and trees
• cortex: the tissue of a stem or root that lies inward from the epidermis, but exterior to the vascular tissue
• parenchyma: the ground tissue making up most of the non-woody parts of a plant | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/35%3A_Plant_Form/35.04%3A_Stems-_Support_for_Above_Ground_Organs/35.4A%3A_Functions_of_Stems.txt |
Learning Objectives
• Distinguish between primary and secondary growth in stems
Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that are woody, also increase in thickness during their life span. The increase in length of the shoot and the root is referred to as primary growth. It is the result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant. It is caused by cell division in the lateral meristem. Herbaceous plants mostly undergo primary growth, with little secondary growth or increase in thickness. Secondary growth, or “wood”, is noticeable in woody plants; it occurs in some dicots, but occurs very rarely in monocots.
Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size.
Primary Growth
Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly-dividing cells in the apical meristems at the shoot tip and root tip. Subsequent cell elongation also contributes to primary growth. The growth of shoots and roots during primary growth enables plants to continuously seek water (roots) or sunlight (shoots).
The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.
Secondary Growth
The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium. The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem ( tracheids and vessel elements) to the inside and secondary phloem (sieve elements and companion cells) to the outside. The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.
In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells (bark) containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels, which allow the interior cells to exchange gases with the outside atmosphere. This supplies oxygen to the living- and metabolically-active cells of the cortex, xylem, and phloem.
Annual Rings
The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter; their primary cell walls are not extensively thickened. This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem. An examination of the number of annual rings and their nature (such as their size and cell wall thickness) can reveal the age of the tree and the prevailing climatic conditions during each season.
Key Points
• Indeterminate growth continues throughout a plant’s life, while determinate growth stops when a plant element (such as a leaf) reaches a particular size.
• Primary growth of stems is a result of rapidly-dividing cells in the apical meristems at the shoot tips.
• Apical dominance reduces the growth along the sides of branches and stems, giving the tree a conical shape.
• The growth of the lateral meristems, which includes the vascular cambium and the cork cambium (in woody plants), increases the thickness of the stem during secondary growth.
• Cork cells (bark) protect the plant against physical damage and water loss; they contain a waxy substance known as suberin that prevents water from penetrating the tissue.
• The secondary xylem develops dense wood during the fall and thin wood during the spring, which produces a characteristic ring for each year of growth.
Key Terms
• lenticel: small, oval, rounded spots upon the stem or branch of a plant that allow the exchange of gases with the surrounding atmosphere
• periderm: the outer layer of plant tissue; the outer bark
• suberin: a waxy material found in bark that can repel water | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/35%3A_Plant_Form/35.04%3A_Stems-_Support_for_Above_Ground_Organs/35.4C%3A_Primary_and_Secondary_Growth_in_Stems.txt |
Learning Objectives
• Explain the reasons for stem modifications
Some plant species have modified stems that are especially suited to a particular habitat and environment. A rhizome is a modified stem that grows horizontally underground; it has nodes and internodes. Vertical shoots may arise from the buds on the rhizome of some plants, such as ginger and ferns. Corms are similar to rhizomes, except they are more rounded and fleshy (such as in gladiolus). Corms contain stored food that enables some plants to survive the winter. Stolons are stems that run almost parallel to the ground, or just below the surface, and can give rise to new plants at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at nodes at varying intervals: strawberries are an example. Tubers are modified stems that may store starch, as seen in the potato. Tubers arise as swollen ends of stolons, and contain many adventitious or unusual buds (familiar to us as the “eyes” on potatoes). A bulb, which functions as an underground storage unit, is a modification of a stem that has the appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem, as seen in the iris.
Modifications to the aerial stems, vegetative buds, and floral buds of plants perform functions such as climbing, protection, and synthesis of food vegetative propagation. Aerial modifications of stems include the following:
• Tendrils are slender, twining strands that enable a plant (like the buckwheat vine) to seek support by climbing on other surfaces. These may develop from either the axillary bud or the terminal bud of the stem.
• Thorns are modified branches appearing as hard, woody, sharp outgrowths that protect the plant; common examples include roses, osage orange, and devil’s walking stick.
• Bulbils are axillary buds that have become fleshy and rounded due to storage of food. They become detached from the plant, fall on ground and develop into a new plant.
• Cladodes are green branches of limited growth (usually one internode long) which have taken up the functions of photosynthesis.
Key Points
• Modified stems that grow horizontally underground are either rhizomes, from which vertical shoots grow, or fleshier, food-storing corms.
• New plants can arise from the nodes of stolons and runners (an aboveground stolon): stems that run parallel to the ground, or just below the surface.
• Potatoes are examples of tubers: the swollen ends of stolons that may store starch.
• The stem modification that has enlarged fleshy leaves emerging from the stem or surrounding the base of the stem is called a bulb; it is also used to store food.
• Aerial modifications of stems include tendrils, thorns, bulbils, and cladodes..
Key Terms
• stolon: a shoot that grows along the ground and produces roots at its nodes; a runner
• tuber: a fleshy, thickened, underground stem of a plant, usually containing stored starch, as for example a potato or arrowroot
• cladode: green branches of limited growth which have taken up the functions of photosynthesis
• rhizome: a horizontal underground stem of some plants that sends out roots and shoots from its nodes
• corm: a short, vertical, swollen underground stem of a plant that serves as a storage organ to enable the plant to survive winter or other adverse conditions such as drought
• bulb: the bulb-shaped root portion of a plant such as a tulip, from which the rest of the plant may be regrown
• tendril: a thin, spirally-coiling stem that attaches a plant to its support
• thorn: a sharp, protective spine of a plant
• bulbil: a bulb-shaped bud in the place of a flower or in a leaf axil | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/35%3A_Plant_Form/35.04%3A_Stems-_Support_for_Above_Ground_Organs/35.4D%3A__Stem_Modifications.txt |
Learning Objectives
• Sketch the basic structure of a typical leaf
Structure of a Typical Leaf
Each leaf typically has a leaf blade called the lamina, which is also the widest part of the leaf. Some leaves are attached to the plant stem by a petiole. Leaves that do not have a petiole and are directly attached to the plant stem are called sessile leaves. Leaves also have stipules, small green appendages usually found at the base of the petiole. Most leaves have a midrib, which travels the length of the leaf and branches to each side to produce veins of vascular tissue. The edge of the leaf is called the margin.
Within each leaf, the vascular tissue forms veins. The arrangement of veins in a leaf is called the venation pattern. Monocots and dicots differ in their patterns of venation. Monocots have parallel venation in which the veins run in straight lines across the length of the leaf without converging. In dicots, however, the veins of the leaf have a net-like appearance, forming a pattern known as reticulate venation. Ginkgo biloba is an example of a plant with dichotomous venation.
Leaf Arrangement
The arrangement of leaves on a stem is known as phyllotaxy. The number and placement of a plant’s leaves will vary depending on the species, with each species exhibiting a characteristic leaf arrangement. Leaves are classified as either alternate, spiral, opposite, or whorled. Plants that have only one leaf per node have leaves that are said to be either alternate or spiral. Alternate leaves alternate on each side of the stem in a flat plane, and spiral leaves are arranged in a spiral along the stem. In an opposite leaf arrangement, two leaves arise at the same point, with the leaves connecting opposite each other along the branch. If there are three or more leaves connected at a node, the leaf arrangement is classified as whorled.
Key Points
• Each leaf typically has a leaf blade ( lamina ), stipules, a midrib, and a margin.
• Some leaves have a petiole, which attaches the leaf to the stem; leaves that do not have petioles are directly attached to the plant stem and are called sessile leaves.
• The arrangement of veins in a leaf is called the venation pattern; monocots have parallel venation, while dicots have reticulate venation.
• The arrangement of leaves on a stem is known as phyllotaxy; leaves can be classified as either alternate, spiral, opposite, or whorled.
• Plants with alternate and spiral leaf arrangements have only one leaf per node.
• In an opposite leaf arrangement, two leaves connect at a node. In a whorled arrangement, three or more leaves connect at a node.
Key Terms
• petiole: stalk that extends from the stem to the base of the leaf
• lamina: the flat part of a leaf; the blade, which is the widest part of the leaf
• stipule: small green appendage usually found at the base of the petiole
35.5B: Types of Leaf Forms
Learning Objectives
• Differentiate among the types of leaf forms
Leaf Form
There are two basic forms of leaves that can be described considering the way the blade (or lamina) is divided. Leaves may be simple or compound.
In a simple leaf, such as the banana leaf, the blade is completely undivided. The leaf shape may also be formed of lobes where the gaps between lobes do not reach to the main vein. An example of this type is the maple leaf.
In a compound leaf, the leaf blade is completely divided, forming leaflets, as in the locust tree. Compound leaves are a characteristic of some families of higher plants. Each leaflet is attached to the rachis (middle vein), but may have its own stalk. A palmately compound leaf has its leaflets radiating outwards from the end of the petiole, like fingers off the palm of a hand. Examples of plants with palmately compound leaves include poison ivy, the buckeye tree, or the familiar house plant Schefflera sp. (commonly called “umbrella plant”). Pinnately compound leaves take their name from their feather-like appearance; the leaflets are arranged along the middle vein, as in rose leaves or the leaves of hickory, pecan, ash, or walnut trees. In a pinnately compound leaf, the middle vein is called the midrib. Bipinnately compound (or double compound) leaves are twice divided; the leaflets are arranged along a secondary vein, which is one of several veins branching off the middle vein. Each leaflet is called a “pinnule”. The pinnules on one secondary vein are called “pinna”. The silk tree (Albizia) is an example of a plant with bipinnate leaves.
Key Points
• In a simple leaf, the blade is completely undivided; leaves may also be formed of lobes where the gaps between lobes do not reach to the main vein.
• In a compound leaf, the leaf blade is divided, forming leaflets that are attached to the middle vein, but have their own stalks.
• The leaflets of palmately-compound leaves radiate outwards from the end of the petiole.
• Pinnately-compound leaves have their leaflets arranged along the middle vein.
• Bipinnately-compound (double-compound) leaves have their leaflets arranged along a secondary vein, which is one of several veins branching off the middle vein.
Key Terms
• simple leaf: a leaf with an undivided blade
• compound leaf: a leaf where the blade is divided, forming leaflets
• palmately compound leaf: leaf that has its leaflets radiating outwards from the end of the petiole
• pinnately compound leaf: a leaf where the leaflets are arranged along the middle vein | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/35%3A_Plant_Form/35.05%3A_Leaves-_Photosynthetic_Organs/35.5A%3A_Leaf_Structure_and_Arrangment.txt |
Learning Objectives
• Describe the internal structure and function of a leaf
Leaf Structure and Function
The outermost layer of the leaf is the epidermis. It consists of the upper and lower epidermis, which are present on either side of the leaf. Botanists call the upper side the adaxial surface (or adaxis) and the lower side the abaxial surface (or abaxis). The epidermis aids in the regulation of gas exchange. It contains stomata, which are openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing. Guard cells are the only epidermal cells to contain chloroplasts.
The epidermis is usually one cell layer thick. However, in plants that grow in very hot or very cold conditions, the epidermis may be several layers thick to protect against excessive water loss from transpiration. A waxy layer known as the cuticle covers the leaves of all plant species. The cuticle reduces the rate of water loss from the leaf surface. Other leaves may have small hairs (trichomes) on the leaf surface. Trichomes help to avert herbivory by restricting insect movements or by storing toxic or bad-tasting compounds. They can also reduce the rate of transpiration by blocking air flow across the leaf surface.
Below the epidermis of dicot leaves are layers of cells known as the mesophyll, or “middle leaf.” The mesophyll of most leaves typically contains two arrangements of parenchyma cells: the palisade parenchyma and spongy parenchyma. The palisade parenchyma (also called the palisade mesophyll) aids in photosynthesis and has column-shaped, tightly-packed cells. It may be present in one, two, or three layers. Below the palisade parenchyma are loosely-arranged cells of an irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll). The air space found between the spongy parenchyma cells allows gaseous exchange between the leaf and the outside atmosphere through the stomata. In aquatic plants, the intercellular spaces in the spongy parenchyma help the leaf float. Both layers of the mesophyll contain many chloroplasts.
Similar to the stem, the leaf contains vascular bundles composed of xylem and phloem. The xylem consists of tracheids and vessels, which transport water and minerals to the leaves. The phloem transports the photosynthetic products from the leaf to the other parts of the plant. A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues.
Leaf Adaptations
Coniferous plant species that thrive in cold environments, such as spruce, fir, and pine, have leaves that are reduced in size and needle-like in appearance. These needle-like leaves have sunken stomata and a smaller surface area, two attributes that aid in reducing water loss. In hot climates, plants such as cacti have succulent leaves that help to conserve water. Many aquatic plants have leaves with wide lamina that can float on the surface of the water; a thick waxy cuticle on the leaf surface that repels water.
Key Points
• The epidermis consists of the upper and lower epidermis; it aids in the regulation of gas exchange via stomata.
• The epidermis is one layer thick, but may have more layers to prevent transpiration.
• The cuticle is located outside the epidermis and protects against water loss; trichomes discourage predation.
• The mesophyll is found between the upper and lower epidermis; it aids in gas exchange and photosynthesis via chloroplasts.
• The xylem transports water and minerals to the leaves; the phloem transports the photosynthetic products to the other parts of the plant.
• Plants in cold climates have needle-like leaves that are reduced in size; plants in hot climates have succulent leaves that help to conserve water.
Key Terms
• trichome: a hair- or scale-like extension of the epidermis of a plant
• cuticle: a noncellular protective covering outside the epidermis of many invertebrates and plants
• mesophyll: the inner tissue (parenchyma) of a leaf, containing many chloroplasts. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/35%3A_Plant_Form/35.05%3A_Leaves-_Photosynthetic_Organs/35.5C%3A__Leaf_Structure_Function_and_Adaptation.txt |
• 36.1: Transport Mechanisms
The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants. To understand how these processes work, we must first understand the energetics of water potential.
• 36.2: Water and Mineral Absorption
• 36.3: Xylem Transport
Most plants secure the water and minerals they need from their roots. The path taken is: soil→roots→stems→leaves soil→roots→stems→leaves. The minerals travel dissolved in the water (often accompanied by various organic molecules supplied by root cells), but less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. Most of it is lost in transpiration, which serve two useful functions: providing the force for lifting the water up the stems and cools the leaves.
• 36.4: Rate of Transpiration
• 36.5: Water-Stress Responses
• 36.6: Phloem Transport
Food and other organic substances (e.g., some plant hormones and even messenger RNAs) manufactured in the cells of the plant are transported in the phloem. Sugars (usually sucrose), amino acids and other organic molecules enter the sieve elements through plasmodesmata connecting them to adjacent companion cells. Once within the sieve elements, these molecules can be transported either up or down to any region of the plant moving at rates as high as 110 μm per second.
36: Transport in Plants
Skills to Develop
• Define water potential and explain how it is influenced by solutes, pressure, gravity, and the matric potential
• Describe how water potential, evapotranspiration, and stomatal regulation influence how water is transported in plants
• Explain how photosynthates are transported in plants
The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants. To understand how these processes work, we must first understand the energetics of water potential.
Water Potential
Plants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a 116-meter-tall tree (Figure $1$a). Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks (Figure $1$b). Plants achieve this because of water potential.
Water potential is a measure of the potential energy in water. Plant physiologists are not interested in the energy in any one particular aqueous system, but are very interested in water movement between two systems. In practical terms, therefore, water potential is the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter ψ (psi) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψwpure H2O) is, by convenience of definition, designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to Ψwpure H2O.
The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. Water potential can be broken down into its individual components using the following equation:
$\psi_\text{system} = \psi_\text{total} = \psi_s + \psi_p + \psi_g + \psi_m \nonumber$
where Ψs, Ψp, Ψg, and Ψm refer to the solute, pressure, gravity, and matric potentials, respectively. “System” can refer to the water potential of the soil water (Ψsoil), root water (Ψroot), stem water (Ψstem), leaf water (Ψleaf) or the water in the atmosphere (Ψatmosphere): whichever aqueous system is under consideration. As the individual components change, they raise or lower the total water potential of a system. When this happens, water moves to equilibrate, moving from the system or compartment with a higher water potential to the system or compartment with a lower water potential. This brings the difference in water potential between the two systems (ΔΨ) back to zero (ΔΨ = 0). Therefore, for water to move through the plant from the soil to the air (a process called transpiration), Ψsoil must be > Ψroot > Ψstem > Ψleaf > Ψatmosphere.
Water only moves in response to ΔΨ, not in response to the individual components. However, because the individual components influence the total Ψsystem, by manipulating the individual components (especially Ψs), a plant can control water movement.
Solute Potential
Solute potential (Ψs), also called osmotic potential, is negative in a plant cell and zero in distilled water. Typical values for cell cytoplasm are –0.5 to –1.0 MPa. Solutes reduce water potential (resulting in a negative Ψw) by consuming some of the potential energy available in the water. Solute molecules can dissolve in water because water molecules can bind to them via hydrogen bonds; a hydrophobic molecule like oil, which cannot bind to water, cannot go into solution. The energy in the hydrogen bonds between solute molecules and water is no longer available to do work in the system because it is tied up in the bond. In other words, the amount of available potential energy is reduced when solutes are added to an aqueous system. Thus, Ψs decreases with increasing solute concentration. Because Ψs is one of the four components of Ψsystem or Ψtotal, a decrease in Ψs will cause a decrease in Ψtotal. The internal water potential of a plant cell is more negative than pure water because of the cytoplasm’s high solute content (Figure $2$). Because of this difference in water potential water will move from the soil into a plant’s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential.
Plant cells can metabolically manipulate Ψs (and by extension, Ψtotal) by adding or removing solute molecules. Therefore, plants have control over Ψtotal via their ability to exert metabolic control over Ψs.
Art Connection
Positive water potential is placed on the left side of the tube by increasing Ψp such that the water level rises on the right side. Could you equalize the water level on each side of the tube by adding solute, and if so, how?
Pressure Potential
Pressure potential (Ψp), also called turgor potential, may be positive or negative (Figure $3$). Because pressure is an expression of energy, the higher the pressure, the more potential energy in a system, and vice versa. Therefore, a positive Ψp (compression) increases Ψtotal, and a negative Ψp (tension) decreases Ψtotal. Positive pressure inside cells is contained by the cell wall, producing turgor pressure. Pressure potentials are typically around 0.6–0.8 MPa, but can reach as high as 1.5 MPa in a well-watered plant. A Ψp of 1.5 MPa equates to 210 pounds per square inch (1.5 MPa x 140 lb in-2 MPa-1 = 210 lb/in-2). As a comparison, most automobile tires are kept at a pressure of 30–34 psi. An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered (Figure $3$). Water is lost from the leaves via transpiration (approaching Ψp = 0 MPa at the wilting point) and restored by uptake via the roots.
A plant can manipulate Ψp via its ability to manipulate Ψs and by the process of osmosis. If a plant cell increases the cytoplasmic solute concentration, Ψs will decline, Ψtotal will decline, the ΔΨ between the cell and the surrounding tissue will decline, water will move into the cell by osmosis, and Ψp will increase. Ψp is also under indirect plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate from the leaf, reducing Ψp and Ψtotal of the leaf and increasing ii between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf.
Gravity Potential
Gravity potential (Ψg) is always negative to zero in a plant with no height. It always removes or consumes potential energy from the system. The force of gravity pulls water downwards to the soil, reducing the total amount of potential energy in the water in the plant (Ψtotal). The taller the plant, the taller the water column, and the more influential Ψg becomes. On a cellular scale and in short plants, this effect is negligible and easily ignored. However, over the height of a tall tree like a giant coastal redwood, the gravitational pull of –0.1 MPa m-1 is equivalent to an extra 1 MPa of resistance that must be overcome for water to reach the leaves of the tallest trees. Plants are unable to manipulate Ψg.
Matric Potential
Matric potential (Ψm) is always negative to zero. In a dry system, it can be as low as –2 MPa in a dry seed, and it is zero in a water-saturated system. The binding of water to a matrix always removes or consumes potential energy from the system. Ψm is similar to solute potential because it involves tying up the energy in an aqueous system by forming hydrogen bonds between the water and some other component. However, in solute potential, the other components are soluble, hydrophilic solute molecules, whereas in Ψm, the other components are insoluble, hydrophilic molecules of the plant cell wall. Every plant cell has a cellulosic cell wall and the cellulose in the cell walls is hydrophilic, producing a matrix for adhesion of water: hence the name matric potential. Ψm is very large (negative) in dry tissues such as seeds or drought-affected soils. However, it quickly goes to zero as the seed takes up water or the soil hydrates. Ψm cannot be manipulated by the plant and is typically ignored in well-watered roots, stems, and leaves.
Movement of Water and Minerals in the Xylem
Solutes, pressure, gravity, and matric potential are all important for the transport of water in plants. Water moves from an area of higher total water potential (higher Gibbs free energy) to an area of lower total water potential. Gibbs free energy is the energy associated with a chemical reaction that can be used to do work. This is expressed as ΔΨ.
Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf–atmosphere interface; it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. This value varies greatly depending on the vapor pressure deficit, which can be negligible at high relative humidity (RH) and substantial at low RH. Water from the roots is pulled up by this tension. At night, when stomata shut and transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem vessels and tracheids, and the cohesion of water molecules to each other. This is called the cohesion–tension theory of sap ascent.
Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis. The wet cell wall is exposed to this leaf internal air space, and the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of the mesophyll cells. This decrease creates a greater tension on the water in the mesophyll cells (Figure $4$), thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Rings in the vessels maintain their tubular shape, much like the rings on a vacuum cleaner hose keep the hose open while it is under pressure. Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant, causing a break termed an embolism in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.
Art Connection
Which of the following statements is false?
1. Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the xylem. Transpiration draws water from the leaf.
2. Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the phloem. Transpiration draws water from the leaf.
3. Water potential decreases from the roots to the top of the plant.
4. Water enters the plants through root hairs and exits through stoma.
Transpiration—the loss of water vapor to the atmosphere through stomata—is a passive process, meaning that metabolic energy in the form of ATP is not required for water movement. The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is tightly controlled.
Control of Transpiration
The atmosphere to which the leaf is exposed drives transpiration, but also causes massive water loss from the plant. Up to 90 percent of the water taken up by roots may be lost through transpiration.
Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water loss.
Plants have evolved over time to adapt to their local environment and reduce transpiration(Figure $5$). Desert plant (xerophytes) and plants that grow on other plants (epiphytes) have limited access to water. Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (mesophytes). Aquatic plants (hydrophytes) also have their own set of anatomical and morphological leaf adaptations.
Xerophytes and epiphytes often have a thick covering of trichomes or of stomata that are sunken below the leaf’s surface. Trichomes are specialized hair-like epidermal cells that secrete oils and substances. These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants.
Transportation of Photosynthates in the Phloem
Plants need an energy source to grow. In seeds and bulbs, food is stored in polymers (such as starch) that are converted by metabolic processes into sucrose for newly developing plants. Once green shoots and leaves are growing, plants are able to produce their own food by photosynthesizing. The products of photosynthesis are called photosynthates, which are usually in the form of simple sugars such as sucrose.
Structures that produce photosynthates for the growing plant are referred to as sources. Sugars produced in sources, such as leaves, need to be delivered to growing parts of the plant via the phloem in a process called translocation. The points of sugar delivery, such as roots, young shoots, and developing seeds, are called sinks. Seeds, tubers, and bulbs can be either a source or a sink, depending on the plant’s stage of development and the season.
The products from the source are usually translocated to the nearest sink through the phloem. For example, the highest leaves will send photosynthates upward to the growing shoot tip, whereas lower leaves will direct photosynthates downward to the roots. Intermediate leaves will send products in both directions, unlike the flow in the xylem, which is always unidirectional (soil to leaf to atmosphere). The pattern of photosynthate flow changes as the plant grows and develops. Photosynthates are directed primarily to the roots early on, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development. They are also directed to tubers for storage.
Translocation: Transport from Source to Sink
Photosynthates, such as sucrose, are produced in the mesophyll cells of photosynthesizing leaves. From there they are translocated through the phloem to where they are used or stored. Mesophyll cells are connected by cytoplasmic channels called plasmodesmata. Photosynthates move through these channels to reach phloem sieve-tube elements (STEs) in the vascular bundles. From the mesophyll cells, the photosynthates are loaded into the phloem STEs. The sucrose is actively transported against its concentration gradient (a process requiring ATP) into the phloem cells using the electrochemical potential of the proton gradient. This is coupled to the uptake of sucrose with a carrier protein called the sucrose-H+ symporter.
Phloem STEs have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap. Companion cells are associated with STEs. They assist with metabolic activities and produce energy for the STEs (Figure $6$).
Once in the phloem, the photosynthates are translocated to the closest sink. Phloem sap is an aqueous solution that contains up to 30 percent sugar, minerals, amino acids, and plant growth regulators. The high percentage of sugar decreases Ψs, which decreases the total water potential and causes water to move by osmosis from the adjacent xylem into the phloem tubes, thereby increasing pressure. This increase in total water potential causes the bulk flow of phloem from source to sink (Figure $7$). Sucrose concentration in the sink cells is lower than in the phloem STEs because the sink sucrose has been metabolized for growth, or converted to starch for storage or other polymers, such as cellulose, for structural integrity. Unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from an area of high concentration to one of low concentration. Water diffuses from the phloem by osmosis and is then transpired or recycled via the xylem back into the phloem sap.
Summary
Water potential (Ψ) is a measure of the difference in potential energy between a water sample and pure water. The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and matric potential. Water potential and transpiration influence how water is transported through the xylem in plants. These processes are regulated by stomatal opening and closing. Photosynthates (mainly sucrose) move from sources to sinks through the plant’s phloem. Sucrose is actively loaded into the sieve-tube elements of the phloem. The increased solute concentration causes water to move by osmosis from the xylem into the phloem. The positive pressure that is produced pushes water and solutes down the pressure gradient. The sucrose is unloaded into the sink, and the water returns to the xylem vessels.
Art Connections
Figure $2$: Positive water potential is placed on the left side of the tube by increasing Ψp such that the water level rises on the right side. Could you equalize the water level on each side of the tube by adding solute, and if so, how?
Answer
Yes, you can equalize the water level by adding the solute to the left side of the tube such that water moves toward the left until the water levels are equal.
Figure $4$: Which of the following statements is false?
1. Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the xylem. Transpiration draws water from the leaf.
2. Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the phloem. Transpiration draws water from the leaf.
3. Water potential decreases from the roots to the top of the plant.
4. Water enters the plants through root hairs and exits through stoma.
Answer
B.
Glossary
cuticle
waxy covering on the outside of the leaf and stem that prevents the loss of water
megapascal (MPa)
pressure units that measure water potential
sink
growing parts of a plant, such as roots and young leaves, which require photosynthate
source
organ that produces photosynthate for a plant
translocation
mass transport of photosynthates from source to sink in vascular plants
transpiration
loss of water vapor to the atmosphere through stomata
water potential (Ψw)
the potential energy of a water solution per unit volume in relation to pure water at atmospheric pressure and ambient temperature | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/36%3A_Transport_in_Plants/36.01%3A_Transport_Mechanisms.txt |
Learning Objectives
• Outline the movement of water and minerals in the xylem
Movement of Water and Minerals in the Xylem
Most plants obtain the water and minerals they need through their roots. The path taken is: soil -> roots -> stems -> leaves. The minerals (e.g., K+, Ca2+) travel dissolved in the water (often accompanied by various organic molecules supplied by root cells). Water and minerals enter the root by separate paths which eventually converge in the stele, or central vascular bundle in roots.
Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf, or atmosphere interface; it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. However, this value varies greatly depending on the vapor pressure deficit, which can be insignificant at high relative humidity (RH) and substantial at low RH. Water from the roots is pulled up by this tension. At night, when stomata close and transpiration stops, the water is held in the stem and leaf by the cohesion of water molecules to each other as well as the adhesion of water to the cell walls of the xylem vessels and tracheids. This is called the cohesion–tension theory of sap ascent.
The cohesion-tension theory explains how water moves up through the xylem. Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis. The wet cell wall is exposed to the internal air space and the water on the surface of the cells evaporates into the air spaces. This decreases the thin film on the surface of the mesophyll cells. The decrease creates a greater tension on the water in the mesophyll cells, thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Small perforations between vessel elements reduce the number and size of gas bubbles that form via a process called cavitation. The formation of gas bubbles in the xylem is detrimental since it interrupts the continuous stream of water from the base to the top of the plant, causing a break (embolism) in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water in a continuous column, increasing the number of cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.
Control of Transpiration
Transpiration is a passive process: metabolic energy in the form of ATP is not required for water movement. The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is tightly controlled. The atmosphere to which the leaf is exposed drives transpiration, but it also causes massive water loss from the plant. Up to 90 percent of the water taken up by roots may be lost through transpiration.
Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water loss.
Plants have evolved over time to adapt to their local environment and reduce transpiration. Desert plant (xerophytes) and plants that grow on other plants ( epiphytes ) have limited access to water. Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (mesophytes). Aquatic plants (hydrophytes) also have their own set of anatomical and morphological leaf adaptations.
Xerophytes and epiphytes often have a thick covering of trichomes or stomata that are sunken below the leaf’s surface. Trichomes are specialized hair-like epidermal cells that secrete oils and other substances. These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants.
Key Points
• The cohesion – tension theory of sap ascent explains how how water is pulled up from the roots to the top of the plant.
• Evaporation from mesophyll cells in the leaves produces a negative water potential gradient that causes water and minerals to move upwards from the roots through the xylem.
• Gas bubbles in the xylem can interrupt the flow of water in the plant, so they must be reduced through small perforations between vessel elements.
• Transpiration is controlled by the opening and closing of stomata in response to environmental cues.
• Stomata must open for photosynthesis and respiration, but when stomata are open, water vapor is lost to the external environment, increasing the rate of transpiration.
• Desert plants and plants with limited water access prevent transpiration and excess water loss by utilizing a thicker cuticle, trichomes, or multiple epidermal layers.
Key Terms
• cohesion–tension theory of sap ascent: explains the process of water flow upwards (against the force of gravity) through the xylem of plants
• cavitation: the formation, in a fluid, of vapor bubbles that can interrupt water flow through the plant
• trichome: a hair- or scale-like extension of the epidermis of a plant | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/36%3A_Transport_in_Plants/36.02%3A_Water_and_Mineral_Absorption/36.2C%3A_Movement_of_Water_and_Minerals_in_the_Xylem.txt |
Most plants secure the water and minerals they need from their roots. The path taken is:
$\text{soil} \rightarrow \text{roots} \rightarrow \text{stems} \rightarrow \text{leaves}$
The minerals (e.g., K+, Ca2+) travel dissolved in the water (often accompanied by various organic molecules supplied by root cells), but less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. Most of it is lost in transpiration, which serve two useful functions- it provides the force for lifting the water up the stems and it cools the leaves. Water and minerals enter the root by separate paths which eventually converge in the stele.
The Pathway of Water and Minerals
Soil water enters the root through its epidermis. It appears that water then travels in both the cytoplasm of root cells - called the symplast (i.e., it crosses the plasma membrane and then passes from cell to cell through plasmodesmata) and in the nonliving parts of the root - called the apoplast (i.e., in the spaces between the cells and in the cells walls themselves. This water has not crossed a plasma membrane. However, the inner boundary of the cortex, the endodermis, is impervious to water because of a band of lignified matrix called the casparian strip. Therefore, to enter the stele, apoplastic water must enter the symplasm of the endodermal cells. From here it can pass by plasmodesmata into the cells of the stele. Once inside the stele, water is again free to move between cells as well as through them. In young roots, water enters directly into the xylem vessels and/or tracheids. These are nonliving conduits so are part of the apoplast.
Once in the xylem, water with the minerals that have been deposited in it (as well as occasional organic molecules supplied by the root tissue) move up in the vessels and tracheids. At any level, the water can leave the xylem and pass laterally to supply the needs of other tissues. At the leaves, the xylem passes into the petiole and then into the veins of the leaf. Water leaves the finest veins and enters the cells of the spongy and palisade layers. Here some of the water may be used in metabolism, but most is lost in transpiration.
Minerals enter the root by active transport into the symplast of epidermal cells and move toward and into the stele through the plasmodesmata connecting the cells. They enter the water in the xylem from the cells of the pericycle (as well as of parenchyma cells surrounding the xylem) through specialized transmembrane channels.
What Forces Water Through the Xylem?
Observations
• The mechanism is based on purely physical forces because the xylem vessels and tracheids are lifeless.
• Roots are not needed. This was demonstrated over a century ago by a German botanist who sawed down a 70-ft (21 meters) oak tree and placed the base of the trunk in a barrel of picric acid solution. The solution was drawn up the trunk, killing nearby tissues as it went.
• However, leaves are needed. When the acid reached the leaves and killed them, the upward movement of water ceased.
• Removing a band of bark from around the trunk - a process called girdling - does not interrupt the upward flow of water. Girdling removes only the phloem, not the xylem, and so the foliage does not wilt. (In due course, however, the roots - and thus the entire plant - will die because the roots cannot receive any of the food manufactured by the leaves.)
Transpiration-Pull
In 1895, the Irish plant physiologists H. H. Dixon and J. Joly proposed that water is pulled up the plant by tension (negative pressure) from above. As we have seen, water is continually being lost from leaves by transpiration. Dixon and Joly believed that the loss of water in the leaves exerts a pull on the water in the xylem ducts and draws more water into the leaf. But even the best vacuum pump can pull water up to a height of only 34 ft (10.4 m) or so. This is because a column of water that high exerts a pressure of ~15 lb/in2 (103 kilopascals, kPa) just counterbalanced by the pressure of the atmosphere. How can water be drawn to the top of a sequoia (the tallest is 370 feet [113 meters] high)? Taking all factors into account, a pull of at least 270 lb/in2 (~1.9 x 103 kPa) is probably needed.
The answer to the dilemma lies the cohesion of water molecules; that is the property of water molecules to cling to each through the hydrogen bonds they form. When ultrapure water is confined to tubes of very small bore, the force of cohesion between water molecules imparts great strength to the column of water. It has been reported that tensions as great as 3000 lb/in2 (21 x 103 kPa) are needed to break the column, about the value needed to break steel wires of the same diameter. In a sense, the cohesion of water molecules gives them the physical properties of solid wires. Because of the critical role of cohesion, the transpiration-pull theory is also called the cohesion theory.
support for Cohesion theory
• If sap in the xylem is under tension, we would expect the column to snap apart if air is introduced into the xylem vessel by puncturing it. This is the case.
• If the water in all the xylem ducts is under tension, there should be a resulting inward pull (because of adhesion) on the walls of the ducts. This inward pull in the band of sapwood in an actively transpiring tree should, in turn, cause a decrease in the diameter of the trunk.
• The graph shows the results of obtained by D. T. MacDougall when he made continuous measurements of the diameter of a Monterey pine. The diameter fluctuated on a daily basis reaching its minimum when the rate of transpiration reached its maximum (around noon)
• The rattan vine may climb as high as 150 ft (45.7 m) on the trees of the tropical rain forest in northeastern Australia to get its foliage into the sun. When the base of a vine is severed while immersed in a basin of water, water continues to be taken up. A vine less than 1 inch (2.5 cm) in diameter will "drink" water indefinitely at a rate of up to 12 ml/minute.
If forced to take water from a sealed container, the vine does so without any decrease in rate, even though the resulting vacuum becomes so great that the remaining water begins to boil spontaneously. (The boiling temperature of water decreases as the air pressure over the water decreases, which is why it takes longer to boil an egg in Denver than in New Orleans.)
• Transpiration-pull enables some trees and shrubs to live in seawater. Seawater is markedly hypertonic to the cytoplasm in the roots of the red mangrove (Rhizophora mangle), and we might expect water to leave the cells resulting in a loss in turgor and wilting. In fact, the remarkably high tensions on the order of 500–800 lb/in2 (~3 to 5 thousand kPa) in the xylem can pull water into the plant against this osmotic gradient. So mangroves literally desalt seawater to meet their needs.
Problems with the theory
When water is placed under a high vacuum, any dissolved gases come out of solution as bubbles (as we saw above with the rattan vine) - this is called cavitation. Any impurities in the water enhance the process. So measurements showing the high tensile strength of water in capillaries require water of high purity - not the case for sap in the xylem. So might cavitation break the column of water in the xylem and thus interrupt its flow? Probably not so long as the tension does not greatly exceed 270 lb/in2 (~1.9 x 103 kPa).
By spinning branches in a centrifuge, it has been shown that water in the xylem avoids cavitation at negative pressures exceeding 225 lb/in2 (~1.6 x 103 kPa). And the fact that sequoias can successfully lift water 358 ft (109 m) - which would require a tension of 270 lb/in2 (~1.9 x 103 kPa) - indicates that cavitation is avoided even at that value. However, such heights may be approaching the limit for xylem transport. Measurements close to the top of the tallest living sequoia (370 ft [=113 m] high) show that the high tensions needed to get water up there have resulted in smaller stomatal openings, causing lower concentrations of CO2 in the needles, causing reduced photosynthesis, causing reduced growth (smaller cells and much smaller needles). (Reported by Koch, G. W. et al., in Nature, 22 April 2004.) So the limits on water transport limit the ultimate height which trees can reach. The tallest tree ever measured, a Douglas fir, was 413 ft. (125.9 meters) high.
Root Pressure
When a tomato plant is carefully severed close to the base of the stem, sap oozes from the stump. The fluid comes out under pressure which is called root pressure. Root pressure is created by the osmotic pressure of xylem sap which is, in turn, created by dissolved minerals and sugars that have been actively transported into the apoplast of the stele.
One important example is the sugar maple when, in very early spring, it hydrolyzes the starches stored in its roots into sugar. This causes water to pass by osmosis through the endodermis and into the xylem ducts. The continuous inflow forces the sap up the ducts.
Although root pressure plays a role in the transport of water in the xylem in some plants and in some seasons, it does not account for most water transport.
• Few plants develop root pressures greater than 30 lb/in2 (207 kPa), and some develop no root pressure at all.
• The volume of fluid transported by root pressure is not enough to account for the measured movement of water in the xylem of most trees and vines.
• Those plants with a reasonably good flow of sap are apt to have the lowest root pressures and vice versa.
• The highest root pressures occur in the spring when the sap is strongly hypertonic to soil water, but the rate of transpiration is low. In summer, when transpiration is high and water is moving rapidly through the xylem, often no root pressure can be detected.
So although root pressure may play a significant role in water transport in certain species (e.g., the coconut palm) or at certain times, most plants meet their needs by transpiration-pull. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/36%3A_Transport_in_Plants/36.03%3A_Xylem_Transport.txt |
Transpiration is the evaporation of water from plants. It occurs chiefly at the leaves while their stomata are open for the passage of CO2 and O2 during photosynthesis. But air that is not fully saturated with water vapor (100% relative humidity) will dry the surfaces of cells with which it comes in contact. So the photosynthesizing leaf loses substantial amount of water by evaporation. This transpired water must be replaced by the transport of more water from the soil to the leaves through the xylem of the roots and stem.
Transpiration is not simply a hazard of plant life. It is the "engine" that pulls water up from the roots to:
• supply photosynthesis (1%-2% of the total)
• bring minerals from the roots for biosynthesis within the leaf
• cool the leaf
Using a potometer (above), one can study the effect of various environmental factors on the rate of transpiration. As water is transpired or otherwise used by the plant, it is replaced from the reservoir on the right. This pushes the air bubble to the left providing a precise measure of the volume of water used.
Environmental factors that affect the rate of transpiration
1. Light
Plants transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of the stomata (mechanism). Light also speeds up transpiration by warming the leaf.
2. Temperature
Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. At 30°C, a leaf may transpire three times as fast as it does at 20°C.
3. Humidity
The rate of diffusion of any substance increases as the difference in concentration of the substances in the two regions increases.When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly.
4. Wind
When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier air.
5. Soil water
A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil. When absorption of water by the roots fails to keep up with the rate of transpiration, loss of turgor occurs, and the stomata close. This immediately reduces the rate of transpiration (as well as of photosynthesis). If the loss of turgor extends to the rest of the leaf and stem, the plant wilts.
The volume of water lost in transpiration can be very high. It has been estimated that over the growing season, one acre of corn (maize) plants may transpire 400,000 gallons (1.5 million liters) of water. As liquid water, this would cover the field with a lake 15 inches (38 cm) deep. An acre of forest probably does even better.
36.04: Rate of Transpiration
Learning Objectives
• Relate the pattern of cell wall thickening in guard cells to their function.
• Explain the mechanism by which blue light triggers stomatal opening.
• Explain the mechanism by which water stress, signaled by abscisic acid, triggers stomatal closure.
Regulation of transpiration is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells (Figure \(1\)). Stomata must open to allow the gas exchange of carbon dioxide and oxygen for efficient photosynthesis (see Photorespiration), and light thus typically triggers stomatal opening. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between gas exchange and water loss. Water stress, high temperatures, and high carbon dioxide concentration causes stomata to close.
Stomatal Opening
Guard cell walls are radially thickened such that the thickenings are concentrated around the stoma (plural: stomata; Figure \(2\)). When turgor pressure increases in guard cells, the cells swell. However, the thickened inner walls near the stoma cannot expand, so they curve to accommodate the expanding outer walls. The curving of the guard cells opens the stoma.
How does light cause stomata to open? Phototropins detect blue light, causing a proton pumps to export protons (H+). ATP, generated by the light reactions of photosynthesis, drives the pump. The cytosol usually more negative than the extracellular solution, and this difference in charge (membrane potential) increases as protons leave the cell. This increase in membrane potential is called hyperpolarization, and it causes potassium (K+) to move down its electrochemical gradient into the cytosol. Protons also move down their electrochemical gradient back into the cytosol, bringing chloride (Cl-) with them through symport channels. Meanwhile, starch is broken down, producing sucrose and malate. Nitrate (NO3-) also enters the cell. The solute potential resulting high concentrations of potassium, chloride, sucrose, malate, and nitrate in the cytosol drives the osmosis of water into the the guard cells. This increases turgor pressure, and the guard cells expand and bend, opening the stoma (Figure \(3\)).
Table \(1\) illustrates how osmotic pressure (which results in turgor pressure) increases with light availability during the day. When the osmotic pressure of the guard cells became greater than that of the surrounding cells, the stomata opened. In the evening, when the osmotic pressure of the guard cells dropped to nearly that of the surrounding cells, the stomata closed.
Table \(1\): Osmotic pressure measured at different times of day in typical guard cells. The osmotic pressure within the other cells of the lower epidermis remained constant at ~1 MPa.
Time Osmotic Pressure (MPa)
7 A.M. 1.46
11 A.M. 3.14
5 P.M. 1.88
12 Midnight 1.32
Stomatal Closure
When water is low, roots synthesize abscisic acid (ABA), which is transported through the xylem to the leaves. There, abscisic acid causes calcium channels to open. Calcium (Ca2+) opens anion channels, and malate, chloride, and nitrate exit the cell. The membrane potential decreases (the difference in charge across the membrane becomes less pronounced) as anions leave the cell. Potassium exits the cell in response to this decrease in membrane potential (called depolarization). The loss of these solutes in the cytosol results in water leaving the cell and a decrease in turgor pressure. The guard cells regain their original shape, and the stoma closes (Figure \(4\)).
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/36%3A_Transport_in_Plants/36.04%3A_Rate_of_Transpiration/36.4.01%3A_Stomatal_Opening_and_Closing.txt |
Learning Objectives
• Identify the locations of synthesis, transport, and actions of abscisic acid.
• Describe how ABA interacts with other plant hormones.
The plant hormone abscisic acid (ABA) was was once thought to be responsible for abscission; however, this is now known to be incorrect. Instead, ABA accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures, or shortened day lengths. Unlike animals, plants cannot flee from potentially harmful conditions like drought, freezing, exposure to salt water or salinated soil, and ABA plays in mediating adaptations of the plant to stress. Abscisic acid (Figure \(1\)) resembles the carotenoid zeaxanthin (Figure \(2\)), from which it is ultimately synthesized. It is produced in mature leaves and roots and transported through the vascular tissue.
Maintaining Dormancy
Seed Maturation and Inhibition of Germination
Seeds are not only important agents of reproduction and dispersal, but they are also essential to the survival of annual and biennial plants. These angiosperms die after flowering and seed formation is complete. Abscisic acid is essential for seed maturation and also enforces a period of seed dormancy, by blocking germination and promoting the synthesis of storage proteins. It is important the seeds not germinate prematurely during unseasonably mild conditions prior to the onset of winter or a dry season. As the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring. As discussed in the Environmental Responses chapter, other environmental cues such as exposure to a cold period, light, or water are often also needed to for germination to occur.
Interestingly, mangrove species with viviparous germination, meaning that seeds germinate while still attached to the parent plant have reduced levels of ABA during embryo formation, providing further evidence of ABA's role in maintain seed dormancy (Farnsworth and Farrant 1998, Am J. Bot.). These mangroves are adapted to drop germinated seeds into surrounding water to be dispersed (Figure \(3\)).
Bud Dormancy
Another effect of ABA is to promote the development of winter buds; it mediates the conversion of the apical meristem into a dormant bud. The newly developing leaves growing above the meristem become converted into stiff bud scales that wrap the meristem closely and will protect it from mechanical damage and drying out during the winter. Abscisic acid in the bud also acts to enforce dormancy so if an unseasonably warm spell occurs before winter is over, the buds will not sprout prematurely. Only after a prolonged period of cold or the lengthening days of spring (photoperiodism) will bud dormancy be lifted.
Response to Water Stress
Stomatal Closure
Abscisic acid also regulates the short-term drought response. Recall that stomata are pores in the leaf and are surrounded by a pair of guard cells. Much of the water taken up by a plant is lost as water vapor exists stomata. Low soil moisture causes an increase in ABA, which causes stomata to close, reducing water loss. Note that stomatal closure also prevents exchange of oxygen and carbon dioxide, which is necessary for efficient photosynthesis (see Photorespiration and Phytosynthetic Pathways). The response to abscisic acid occurs even if blue light is present; that is, signaling from drought via ABA overrides the signaling from blue light to open stomata. See Transport for more details about stomatal opening and closure.
Cellular Protection from Dehydration
Abscisic acid turns on the expression of genes encoding proteins that protect cells - in seeds as well as in vegetative tissues - from damage when they become dehydrated.
Interactions with Other Hormones
At a cellular level, abscisic acid inhibits both cell division and cell expansion. It often opposes the growth-inducing effects of auxin and gibberellic acid. For example, abscisic acid prevents stem elongation probably by its inhibitory effect on gibberellic acid. In maintaining apical dominance, however, ABA synergizes with auxin. Abscisic acid moves up from the roots to the stem (opposite the flow of auxin) and suppresses the development of axillary buds. The result is inhibition of branching (maintaining apical dominance).
Attributions
Curated and authored by Melissa Ha from the following sources: | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/36%3A_Transport_in_Plants/36.05%3A_Water-Stress_Responses/36.5.01%3A_Response_to_Water_Stress.txt |
Food and other organic substances (e.g., some plant hormones and even messenger RNAs) manufactured in the cells of the plant are transported in the phloem. Sugars (usually sucrose), amino acids and other organic molecules enter the sieve elements through plasmodesmata connecting them to adjacent companion cells. Once within the sieve elements, these molecules can be transported either up or down to any region of the plant moving at rates as high as 110 μm per second.
Two demonstrations:
• Girdling. Girdling is removing a band of bark from the circumference of the tree. Girdling removes the phloem, but not the xylem. If a tree is girdled in summer, it continues to live for a time. There is, however, no increase in the weight of the roots, and the bark just above the girdled region accumulates carbohydrates. Unless a special graft is made to bridge the gap, the tree eventually dies as its roots starve.
• The pictures below are autoradiographs showing that the products of photosynthesis are transported in the phloem.
A cucumber leaf was supplied with radioactive water (3HOH) and allowed to carry on photosynthesis for 30 minutes. Then slices were cut from the petiole of the leaf and covered with a photographic emulsion. Radioactive products of photosynthesis darkened the emulsion where it was in contact with the phloem (upper left in both photos), but not where it was in contact with the xylem vessels (center). In the photomicrograph on the left, the microscope is focused on the tissue in order to show the cells clearly; on the right, the microscope has been focused on the photographic emulsion.
Some fruits, such as the pumpkin, receive over 0.5 gram of food each day through the phloem. Because the fluid is fairly dilute, this requires a substantial flow. In fact, the use of radioactive tracers shows that substances can travel through as much as 100 cm of phloem in an hour.
Mechanism that drives translocation of food through the phloem
Translocation through the phloem is dependent on metabolic activity of the phloem cells (in contrast to transport in the xylem).
• Chilling its petiole slows the rate at which food is translocated out of the leaf (above).
• Oxygen lack also depresses it.
• Killing the phloem cells puts an end to it.
The Pressure-Flow Hypothesis
The best-supported theory to explain the movement of food through the phloem is called the pressure-flow hypothesis.
• It proposes that water containing food molecules flows under pressure through the phloem.
• The pressure is created by the difference in water concentration of the solution in the phloem and the relatively pure water in the nearby xylem ducts.
• At their "source" - the leaves - sugars are pumped by active transport into the companion cells and sieve elements of the phloem.
• As sugars (and other products of photosynthesis) accumulate in the phloem, water enters by osmosis.
In the figure, sugar molecules are represented in black, water molecules in red.)
• Turgor pressure builds up in the sieve elements (similar to the creation of root pressure).
• As the fluid is pushed down (and up) the phloem, sugars are removed by the cortex cells of both stem and root (the "sinks") and consumed or converted into starch.
• Starch is insoluble and exerts no osmotic effect.
• Therefore, the osmotic pressure of the contents of the phloem decreases.
• Finally, relatively pure water is left in the phloem, and this leaves by osmosis and/or is drawn back into nearby xylem vessels by the suction of transpiration-pull.
Thus it is the pressure gradient between "source" (leaves) and "sink" (shoot and roots) that drives the contents of the phloem up and down through the sieve elements.
Tests of the theory
1. The contents of the sieve elements must be under pressure.
This is difficult to measure because when a sieve element is punctured with a measuring probe, the holes in its end walls quickly plug up. However, aphids can insert their mouth parts without triggering this response.
Left: when it punctures a sieve element, sap enters the insect's mouth parts under pressure and some soon emerges at the other end (as a drop of honeydew that serves as food for ants and bees).
Right: honeydew will continue to exude from the mouthparts after the aphid has been cut away from them.
2. The osmotic pressure of the fluid in the phloem of the leaves must be greater than that in the phloem of the food-receiving organs such as the roots and fruits. Most measurements have shown this to be true.
Transport of Messenger RNA (mRNA) through the Phloem
Plant scientists at the Davis campus of the University of California (reported in the 13 July 2001 issue of Science) have demonstrated that messenger RNAs can also be transported long distances in the phloem. They grafted normal tomato scions onto mutant tomato stocks and found that mRNAs synthesized in the stock were transported into the scions. These mRNAs converted the phenotype of the scion into that of the stock. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/36%3A_Transport_in_Plants/36.06%3A_Phloem_Transport.txt |
• 37.1: Soils- The Substrates on Which Plants Depend
Soil is the outer loose layer that covers the surface of Earth. Soil quality is a major determinant, along with climate, of plant distribution and growth. Soil quality depends not only on the chemical composition of the soil, but also the topography (regional surface features) and the presence of living organisms. In agriculture, the history of the soil, such as the cultivating practices and previous crops, modify the characteristics and fertility of that soil.
• 37.2: Plant Nutrients
Plants are unique organisms that can absorb nutrients and water through their root system, as well as carbon dioxide from the atmosphere. Soil quality and climate are the major determinants of plant distribution and growth. The combination of soil nutrients, water, and carbon dioxide, along with sunlight, allows plants to grow.
• 37.3: Special Nutritional Strategies
Plants obtain food in two different ways. Autotrophic plants can make their own food from inorganic raw materials, such as carbon dioxide and water, through photosynthesis in the presence of sunlight. Green plants are included in this group. Some plants, however, are heterotrophic: they are totally parasitic and lacking in chlorophyll. These plants, referred to as holo-parasitic plants, are unable to synthesize organic carbon and draw all of their nutrients from the host plant.
• 37.4: Carbon-Nitrogen Balance and Global Change
• 37.5: Phytoremediation
37: Plant Nutrition and Soils
Skills to Develop
• Describe how soils are formed
• Explain soil composition
• Describe a soil profile
Plants obtain inorganic elements from the soil, which serves as a natural medium for land plants. Soil is the outer loose layer that covers the surface of Earth. Soil quality is a major determinant, along with climate, of plant distribution and growth. Soil quality depends not only on the chemical composition of the soil, but also the topography (regional surface features) and the presence of living organisms. In agriculture, the history of the soil, such as the cultivating practices and previous crops, modify the characteristics and fertility of that soil.
Soil develops very slowly over long periods of time, and its formation results from natural and environmental forces acting on mineral, rock, and organic compounds. Soils can be divided into two groups: organic soils are those that are formed from sedimentation and primarily composed of organic matter, while those that are formed from the weathering of rocks and are primarily composed of inorganic material are called mineral soils. Mineral soils are predominant in terrestrial ecosystems, where soils may be covered by water for part of the year or exposed to the atmosphere.
Soil Composition
Soil consists of these major components (Figure \(1\)):
• inorganic mineral matter, about 40 to 45 percent of the soil volume
• organic matter, about 5 percent of the soil volume
• water and air, about 50 percent of the soil volume
The amount of each of the four major components of soil depends on the amount of vegetation, soil compaction, and water present in the soil. A good healthy soil has sufficient air, water, minerals, and organic material to promote and sustain plant life.
Exercise \(1\)
Soil compaction can result when soil is compressed by heavy machinery or even foot traffic. How might this compaction change the soil composition?
Answer
The air content of the soil decreases.
The organic material of soil, called humus, is made up of microorganisms (dead and alive), and dead animals and plants in varying stages of decay. Humus improves soil structure and provides plants with water and minerals. The inorganic material of soil consists of rock, slowly broken down into smaller particles that vary in size. Soil particles that are 0.1 to 2 mm in diameter are sand. Soil particles between 0.002 and 0.1 mm are called silt, and even smaller particles, less than 0.002 mm in diameter, are called clay. Some soils have no dominant particle size and contain a mixture of sand, silt, and humus; these soils are called loams.
Soil Formation
Soil formation is the consequence of a combination of biological, physical, and chemical processes. Soil should ideally contain 50 percent solid material and 50 percent pore space. About one-half of the pore space should contain water, and the other half should contain air. The organic component of soil serves as a cementing agent, returns nutrients to the plant, allows soil to store moisture, makes soil tillable for farming, and provides energy for soil microorganisms. Most soil microorganisms—bacteria, algae, or fungi—are dormant in dry soil, but become active once moisture is available.
Soil distribution is not homogenous because its formation results in the production of layers; together, the vertical section of a soil is called the soil profile. Within the soil profile, soil scientists define zones called horizons. A horizon is a soil layer with distinct physical and chemical properties that differ from those of other layers. Five factors account for soil formation: parent material, climate, topography, biological factors, and time.
Parent Material
The organic and inorganic material in which soils form is the parent material. Mineral soils form directly from the weathering of bedrock, the solid rock that lies beneath the soil, and therefore, they have a similar composition to the original rock. Other soils form in materials that came from elsewhere, such as sand and glacial drift. Materials located in the depth of the soil are relatively unchanged compared with the deposited material. Sediments in rivers may have different characteristics, depending on whether the stream moves quickly or slowly. A fast-moving river could have sediments of rocks and sand, whereas a slow-moving river could have fine-textured material, such as clay.
Climate
Temperature, moisture, and wind cause different patterns of weathering and therefore affect soil characteristics. The presence of moisture and nutrients from weathering will also promote biological activity: a key component of a quality soil.
Topography
Regional surface features (familiarly called “the lay of the land”) can have a major influence on the characteristics and fertility of a soil. Topography affects water runoff, which strips away parent material and affects plant growth. Steeps soils are more prone to erosion and may be thinner than soils that are relatively flat or level.
Biological factors
The presence of living organisms greatly affects soil formation and structure. Animals and microorganisms can produce pores and crevices, and plant roots can penetrate into crevices to produce more fragmentation. Plant secretions promote the development of microorganisms around the root, in an area known as the rhizosphere. Additionally, leaves and other material that fall from plants decompose and contribute to soil composition.
Time
Time is an important factor in soil formation because soils develop over long periods. Soil formation is a dynamic process. Materials are deposited over time, decompose, and transform into other materials that can be used by living organisms or deposited onto the surface of the soil.
Physical Properties of the Soil
Soils are named and classified based on their horizons. The soil profile has four distinct layers: 1) O horizon; 2) A horizon; 3) B horizon, or subsoil; and 4) C horizon, or soil base (Figure \(2\)). The O horizon has freshly decomposing organic matter—humus—at its surface, with decomposed vegetation at its base. Humus enriches the soil with nutrients and enhances soil moisture retention. Topsoil—the top layer of soil—is usually two to three inches deep, but this depth can vary considerably. For instance, river deltas like the Mississippi River delta have deep layers of topsoil. Topsoil is rich in organic material; microbial processes occur there, and it is the “workhorse” of plant production. The A horizon consists of a mixture of organic material with inorganic products of weathering, and it is therefore the beginning of true mineral soil. This horizon is typically darkly colored because of the presence of organic matter. In this area, rainwater percolates through the soil and carries materials from the surface. The B horizon is an accumulation of mostly fine material that has moved downward, resulting in a dense layer in the soil. In some soils, the B horizon contains nodules or a layer of calcium carbonate. The C horizon, or soil base, includes the parent material, plus the organic and inorganic material that is broken down to form soil. The parent material may be either created in its natural place, or transported from elsewhere to its present location. Beneath the C horizon lies bedrock.
Exercise \(2\)
Which horizon is considered the topsoil, and which is considered the subsoil?
Answer
The A horizon is the topsoil, and the B horizon is subsoil.
Some soils may have additional layers, or lack one of these layers. The thickness of the layers is also variable, and depends on the factors that influence soil formation. In general, immature soils may have O, A, and C horizons, whereas mature soils may display all of these, plus additional layers (Figure \(3\)).
Career Connections: Soil Scientist
A soil scientist studies the biological components, physical and chemical properties, distribution, formation, and morphology of soils. Soil scientists need to have a strong background in physical and life sciences, plus a foundation in mathematics. They may work for federal or state agencies, academia, or the private sector. Their work may involve collecting data, carrying out research, interpreting results, inspecting soils, conducting soil surveys, and recommending soil management programs.
Many soil scientists work both in an office and in the field. According to the United States Department of Agriculture (USDA): “a soil scientist needs good observation skills to analyze and determine the characteristics of different types of soils. Soil types are complex and the geographical areas a soil scientist may survey are varied. Aerial photos or various satellite images are often used to research the areas. Computer skills and geographic information systems (GIS) help the scientist to analyze the multiple facets of geomorphology, topography, vegetation, and climate to discover the patterns left on the landscape.”1 Soil scientists play a key role in understanding the soil’s past, analyzing present conditions, and making recommendations for future soil-related practices.
Summary
Plants obtain mineral nutrients from the soil. Soil is the outer loose layer that covers the surface of Earth. Soil quality depends on the chemical composition of the soil, the topography, the presence of living organisms, the climate, and time. Agricultural practice and history may also modify the characteristics and fertility of soil. Soil consists of four major components: 1) inorganic mineral matter, 2) organic matter, 3) water and air, and 4) living matter. The organic material of soil is made of humus, which improves soil structure and provides water and minerals. Soil inorganic material consists of rock slowly broken down into smaller particles that vary in size, such as sand, silt, and loam.
Soil formation results from a combination of biological, physical, and chemical processes. Soil is not homogenous because its formation results in the production of layers called a soil profile. Factors that affect soil formation include: parent material, climate, topography, biological factors, and time. Soils are classified based on their horizons, soil particle size, and proportions. Most soils have four distinct horizons: O, A, B, and C.
Footnotes
1. 1 National Resources Conservation Service / United States Department of Agriculture. “Careers in Soil Science.” soils.usda.gov/education/facts/careers.html
Glossary
A horizon
consists of a mixture of organic material with inorganic products of weathering
B horizon
soil layer that is an accumulation of mostly fine material that has moved downward
bedrock
solid rock that lies beneath the soil
C horizon
layer of soil that contains the parent material, and the organic and inorganic material that is broken down to form soil; also known as the soil base
clay
soil particles that are less than 0.002 mm in diameter
horizon
soil layer with distinct physical and chemical properties, which differs from other layers depending on how and when it was formed
humus
organic material of soil; made up of microorganisms, dead animals and plants in varying stages of decay
loam
soil that has no dominant particle size
mineral soil
type of soil that is formed from the weathering of rocks and inorganic material; composed primarily of sand, silt, and clay
O horizon
layer of soil with humus at the surface and decomposed vegetation at the base
organic soil
type of soil that is formed from sedimentation; composed primarily of organic material
parent material
organic and inorganic material in which soils form
rhizosphere
area of soil affected by root secretions and microorganisms
sand
soil particles between 0.1–2 mm in diameter
silt
soil particles between 0.002 and 0.1 mm in diameter
soil profile
vertical section of a soil
soil
outer loose layer that covers the surface of Earth | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/37%3A_Plant_Nutrition_and_Soils/37.01%3A_Soils-_The_Substrates_on_Which_Plants_Depend.txt |
Skills to Develop
• Describe how plants obtain nutrients
• List the elements and compounds required for proper plant nutrition
• Describe an essential nutrient
Plants are unique organisms that can absorb nutrients and water through their root system, as well as carbon dioxide from the atmosphere. Soil quality and climate are the major determinants of plant distribution and growth. The combination of soil nutrients, water, and carbon dioxide, along with sunlight, allows plants to grow.
The Chemical Composition of Plants
Since plants require nutrients in the form of elements such as carbon and potassium, it is important to understand the chemical composition of plants. The majority of volume in a plant cell is water; it typically comprises 80 to 90 percent of the plant’s total weight. Soil is the water source for land plants, and can be an abundant source of water, even if it appears dry. Plant roots absorb water from the soil through root hairs and transport it up to the leaves through the xylem. As water vapor is lost from the leaves, the process of transpiration and the polarity of water molecules (which enables them to form hydrogen bonds) draws more water from the roots up through the plant to the leaves (Figure \(1\)). Plants need water to support cell structure, for metabolic functions, to carry nutrients, and for photosynthesis.
Plant cells need essential substances, collectively called nutrients, to sustain life. Plant nutrients may be composed of either organic or inorganic compounds. An organic compound is a chemical compound that contains carbon, such as carbon dioxide obtained from the atmosphere. Carbon that was obtained from atmospheric CO2 composes the majority of the dry mass within most plants. An inorganic compound does not contain carbon and is not part of, or produced by, a living organism. Inorganic substances, which form the majority of the soil solution, are commonly called minerals: those required by plants include nitrogen (N) and potassium (K) for structure and regulation.
Essential Nutrients
Plants require only light, water and about 20 elements to support all their biochemical needs: these 20 elements are called essential nutrients. For an element to be regarded as essential, three criteria are required: 1) a plant cannot complete its life cycle without the element; 2) no other element can perform the function of the element; and 3) the element is directly involved in plant nutrition.
Table \(1\): Essential elements for plant growth
Macronutrients Micronutrients
Carbon (C) Iron (Fe)
Hydrogen (H) Manganese (Mn)
Oxygen (O) Boron (B)
Nitrogen (N) Molybdenum (Mo)
Phosphorus (P) Copper (Cu)
Potassium (K) Zinc (Zn)
Calcium (Ca) Chlorine (Cl)
Magnesium (Mg) Nickel (Ni)
Sulfur (S) Cobalt (Co)
Sodium (S)
Silicon (Si)
Macronutrients and Micronutrients
The essential elements can be divided into two groups: macronutrients and micronutrients. Nutrients that plants require in larger amounts are called macronutrients. About half of the essential elements are considered macronutrients: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. The first of these macronutrients, carbon (C), is required to form carbohydrates, proteins, nucleic acids, and many other compounds; it is therefore present in all macromolecules. On average, the dry weight (excluding water) of a cell is 50 percent carbon. As shown in Figure \(2\), carbon is a key part of plant biomolecules.
The next most abundant element in plant cells is nitrogen (N); it is part of proteins and nucleic acids. Nitrogen is also used in the synthesis of some vitamins. Hydrogen and oxygen are macronutrients that are part of many organic compounds, and also form water. Oxygen is necessary for cellular respiration; plants use oxygen to store energy in the form of ATP. Phosphorus (P), another macromolecule, is necessary to synthesize nucleic acids and phospholipids. As part of ATP, phosphorus enables food energy to be converted into chemical energy through oxidative phosphorylation. Likewise, light energy is converted into chemical energy during photophosphorylation in photosynthesis, and into chemical energy to be extracted during respiration. Sulfur is part of certain amino acids, such as cysteine and methionine, and is present in several coenzymes. Sulfur also plays a role in photosynthesis as part of the electron transport chain, where hydrogen gradients play a key role in the conversion of light energy into ATP. Potassium (K) is important because of its role in regulating stomatal opening and closing. As the openings for gas exchange, stomata help maintain a healthy water balance; a potassium ion pump supports this process.
Magnesium (Mg) and calcium (Ca) are also important macronutrients. The role of calcium is twofold: to regulate nutrient transport, and to support many enzyme functions. Magnesium is important to the photosynthetic process. These minerals, along with the micronutrients, which are described below, also contribute to the plant’s ionic balance.
In addition to macronutrients, organisms require various elements in small amounts. These micronutrients, or trace elements, are present in very small quantities. They include boron (B), chlorine (Cl), manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), silicon (Si), and sodium (Na).
Deficiencies in any of these nutrients—particularly the macronutrients—can adversely affect plant growth (Figure \(3\)). Depending on the specific nutrient, a lack can cause stunted growth, slow growth, or chlorosis (yellowing of the leaves). Extreme deficiencies may result in leaves showing signs of cell death.
Everyday Connection: Hydroponics
Hydroponics is a method of growing plants in a water-nutrient solution instead of soil. Since its advent, hydroponics has developed into a growing process that researchers often use. Scientists who are interested in studying plant nutrient deficiencies can use hydroponics to study the effects of different nutrient combinations under strictly controlled conditions. Hydroponics has also developed as a way to grow flowers, vegetables, and other crops in greenhouse environments. You might find hydroponically grown produce at your local grocery store. Today, many lettuces and tomatoes in your market have been hydroponically grown.
Summary
Plants can absorb inorganic nutrients and water through their root system, and carbon dioxide from the environment. The combination of organic compounds, along with water, carbon dioxide, and sunlight, produce the energy that allows plants to grow. Inorganic compounds form the majority of the soil solution. Plants access water though the soil. Water is absorbed by the plant root, transports nutrients throughout the plant, and maintains the structure of the plant. Essential elements are indispensable elements for plant growth. They are divided into macronutrients and micronutrients. The macronutrients plants require are carbon, nitrogen, hydrogen, oxygen, phosphorus, potassium, calcium, magnesium, and sulfur. Important micronutrients include iron, manganese, boron, molybdenum, copper, zinc, chlorine, nickel, cobalt, silicon and sodium.
Glossary
inorganic compound
chemical compound that does not contain carbon; it is not part of or produced by a living organism
macronutrient
nutrient that is required in large amounts for plant growth; carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur
micronutrient
nutrient required in small amounts; also called trace element
organic compound
chemical compound that contains carbon | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/37%3A_Plant_Nutrition_and_Soils/37.02%3A_Plant_Nutrients.txt |
Skills to Develop
• Understand the nutritional adaptations of plants
• Describe mycorrhizae
• Explain nitrogen fixation
Plants obtain food in two different ways. Autotrophic plants can make their own food from inorganic raw materials, such as carbon dioxide and water, through photosynthesis in the presence of sunlight. Green plants are included in this group. Some plants, however, are heterotrophic: they are totally parasitic and lacking in chlorophyll. These plants, referred to as holo-parasitic plants, are unable to synthesize organic carbon and draw all of their nutrients from the host plant.
Plants may also enlist the help of microbial partners in nutrient acquisition. Particular species of bacteria and fungi have evolved along with certain plants to create a mutualistic symbiotic relationship with roots. This improves the nutrition of both the plant and the microbe. The formation of nodules in legume plants and mycorrhization can be considered among the nutritional adaptations of plants. However, these are not the only type of adaptations that we may find; many plants have other adaptations that allow them to thrive under specific conditions.
Nitrogen Fixation: Root and Bacteria Interactions
Nitrogen is an important macronutrient because it is part of nucleic acids and proteins. Atmospheric nitrogen, which is the diatomic molecule $\ce{N2}$, or dinitrogen, is the largest pool of nitrogen in terrestrial ecosystems. However, plants cannot take advantage of this nitrogen because they do not have the necessary enzymes to convert it into biologically useful forms. However, nitrogen can be “fixed,” which means that it can be converted to ammonia ($\ce{NH3}$) through biological, physical, or chemical processes. As you have learned, biological nitrogen fixation (BNF) is the conversion of atmospheric nitrogen ($\ce{N2}$) into ammonia ($\ce{NH3}$), exclusively carried out by prokaryotes such as soil bacteria or cyanobacteria. Biological processes contribute 65 percent of the nitrogen used in agriculture. The following equation represents the process:
$\ce { N2 + 16 ATP + 8 e^{-} + 8 H^{+} \rightarrow 2 NH3 + 16 ADP + 16 P_i + H_2} \nonumber$
The most important source of BNF is the symbiotic interaction between soil bacteria and legume plants, including many crops important to humans (Figure $1$). The NH3 resulting from fixation can be transported into plant tissue and incorporated into amino acids, which are then made into plant proteins. Some legume seeds, such as soybeans and peanuts, contain high levels of protein, and serve among the most important agricultural sources of protein in the world.
Exercise $1$
Farmers often rotate corn (a cereal crop) and soy beans (a legume) planting a field with each crop in alternate seasons. What advantage might this crop rotation confer?
Answer
Soybeans are able to fix nitrogen in their roots, which are not harvested at the end of the growing season. The belowground nitrogen can be used in the next season by the corn.
Farmers often rotate corn (a cereal crop) and soy beans (a legume), planting a field with each crop in alternate seasons. What advantage might this crop rotation confer?
Soil bacteria, collectively called rhizobia, symbiotically interact with legume roots to form specialized structures called nodules, in which nitrogen fixation takes place. This process entails the reduction of atmospheric nitrogen to ammonia, by means of the enzyme nitrogenase. Therefore, using rhizobia is a natural and environmentally friendly way to fertilize plants, as opposed to chemical fertilization that uses a nonrenewable resource, such as natural gas. Through symbiotic nitrogen fixation, the plant benefits from using an endless source of nitrogen from the atmosphere. The process simultaneously contributes to soil fertility because the plant root system leaves behind some of the biologically available nitrogen. As in any symbiosis, both organisms benefit from the interaction: the plant obtains ammonia, and bacteria obtain carbon compounds generated through photosynthesis, as well as a protected niche in which to grow (Figure $2$).
Mycorrhizae: The Symbiotic Relationship between Fungi and Roots
A nutrient depletion zone can develop when there is rapid soil solution uptake, low nutrient concentration, low diffusion rate, or low soil moisture. These conditions are very common; therefore, most plants rely on fungi to facilitate the uptake of minerals from the soil. Fungi form symbiotic associations called mycorrhizae with plant roots, in which the fungi actually are integrated into the physical structure of the root. The fungi colonize the living root tissue during active plant growth.
Through mycorrhization, the plant obtains mainly phosphate and other minerals, such as zinc and copper, from the soil. The fungus obtains nutrients, such as sugars, from the plant root (Figure $3$). Mycorrhizae help increase the surface area of the plant root system because hyphae, which are narrow, can spread beyond the nutrient depletion zone. Hyphae can grow into small soil pores that allow access to phosphorus that would otherwise be unavailable to the plant. The beneficial effect on the plant is best observed in poor soils. The benefit to fungi is that they can obtain up to 20 percent of the total carbon accessed by plants. Mycorrhizae functions as a physical barrier to pathogens. It also provides an induction of generalized host defense mechanisms, and sometimes involves production of antibiotic compounds by the fungi.
There are two types of mycorrhizae: ectomycorrhizae and endomycorrhizae. Ectomycorrhizae form an extensive dense sheath around the roots, called a mantle. Hyphae from the fungi extend from the mantle into the soil, which increases the surface area for water and mineral absorption. This type of mycorrhizae is found in forest trees, especially conifers, birches, and oaks. Endomycorrhizae, also called arbuscular mycorrhizae, do not form a dense sheath over the root. Instead, the fungal mycelium is embedded within the root tissue. Endomycorrhizae are found in the roots of more than 80 percent of terrestrial plants.
Nutrients from Other Sources
Some plants cannot produce their own food and must obtain their nutrition from outside sources. This may occur with plants that are parasitic or saprophytic. Some plants are mutualistic symbionts, epiphytes, or insectivorous.
Plant Parasites
A parasitic plant depends on its host for survival. Some parasitic plants have no leaves. An example of this is the dodder (Figure $4$), which has a weak, cylindrical stem that coils around the host and forms suckers. From these suckers, cells invade the host stem and grow to connect with the vascular bundles of the host. The parasitic plant obtains water and nutrients through these connections. The plant is a total parasite (a holoparasite) because it is completely dependent on its host. Other parasitic plants (hemiparasites) are fully photosynthetic and only use the host for water and minerals. There are about 4,100 species of parasitic plants.
Saprophytes
A saprophyte is a plant that does not have chlorophyll and gets its food from dead matter, similar to bacteria and fungi (note that fungi are often called saprophytes, which is incorrect, because fungi are not plants). Plants like these use enzymes to convert organic food materials into simpler forms from which they can absorb nutrients (Figure $5$). Most saprophytes do not directly digest dead matter: instead, they parasitize fungi that digest dead matter, or are mycorrhizal, ultimately obtaining photosynthate from a fungus that derived photosynthate from its host. Saprophytic plants are uncommon; only a few species are described.
Symbionts
A symbiont is a plant in a symbiotic relationship, with special adaptations such as mycorrhizae or nodule formation. Fungi also form symbiotic associations with cyanobacteria and green algae (called lichens). Lichens can sometimes be seen as colorful growths on the surface of rocks and trees (Figure $6$). The algal partner (phycobiont) makes food autotrophically, some of which it shares with the fungus; the fungal partner (mycobiont) absorbs water and minerals from the environment, which are made available to the green alga. If one partner was separated from the other, they would both die.
Epiphytes
An epiphyte is a plant that grows on other plants, but is not dependent upon the other plant for nutrition (Figure $7$). Epiphytes have two types of roots: clinging aerial roots, which absorb nutrients from humus that accumulates in the crevices of trees; and aerial roots, which absorb moisture from the atmosphere.
Insectivorous Plants
An insectivorous plant has specialized leaves to attract and digest insects. The Venus flytrap is popularly known for its insectivorous mode of nutrition, and has leaves that work as traps (Figure $8$). The minerals it obtains from prey compensate for those lacking in the boggy (low pH) soil of its native North Carolina coastal plains. There are three sensitive hairs in the center of each half of each leaf. The edges of each leaf are covered with long spines. Nectar secreted by the plant attracts flies to the leaf. When a fly touches the sensory hairs, the leaf immediately closes. Next, fluids and enzymes break down the prey and minerals are absorbed by the leaf. Since this plant is popular in the horticultural trade, it is threatened in its original habitat.
Summary
Atmospheric nitrogen is the largest pool of available nitrogen in terrestrial ecosystems. However, plants cannot use this nitrogen because they do not have the necessary enzymes. Biological nitrogen fixation (BNF) is the conversion of atmospheric nitrogen to ammonia. The most important source of BNF is the symbiotic interaction between soil bacteria and legumes. The bacteria form nodules on the legume’s roots in which nitrogen fixation takes place. Fungi form symbiotic associations (mycorrhizae) with plants, becoming integrated into the physical structure of the root. Through mycorrhization, the plant obtains minerals from the soil and the fungus obtains photosynthate from the plant root. Ectomycorrhizae form an extensive dense sheath around the root, while endomycorrhizae are embedded within the root tissue. Some plants—parasites, saprophytes, symbionts, epiphytes, and insectivores—have evolved adaptations to obtain their organic or mineral nutrition from various sources.
Glossary
epiphyte
plant that grows on other plants but is not dependent upon other plants for nutrition
insectivorous plant
plant that has specialized leaves to attract and digest insects
nitrogenase
enzyme that is responsible for the reduction of atmospheric nitrogen to ammonia
nodules
specialized structures that contain Rhizobia bacteria where nitrogen fixation takes place
parasitic plant
plant that is dependent on its host for survival
rhizobia
soil bacteria that symbiotically interact with legume roots to form nodules and fix nitrogen
saprophyte
plant that does not have chlorophyll and gets its food from dead matter
symbiont
plant in a symbiotic relationship with bacteria or fungi | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/37%3A_Plant_Nutrition_and_Soils/37.03%3A_Special_Nutritional_Strategies.txt |
Bioremediation is a waste management technique that involves the use of organisms such as plants, bacteria, and fungi to remove or neutralize pollutants from a contaminated site. According to the United States EPA, bioremediation is a “treatment that uses naturally occurring organisms to break down hazardous substances into less toxic or non toxic substances”.
Bioremediation is widely used to treat human sewage and has also been used to remove agricultural chemicals (pesticides and fertilizers) that leach from soil into groundwater. Certain toxic metals, such as selenium and arsenic compounds, can also be removed from water by bioremediation. Mercury is an example of a toxic metal that can be removed from an environment by bioremediation. Mercury is an active ingredient of some pesticides and is also a byproduct of certain industries, such as battery production. Mercury is usually present in very low concentrations in natural environments but it is highly toxic because it accumulates in living tissues. Several species of bacteria can carry out the biotransformation of toxic mercury into nontoxic forms. These bacteria, such as Pseudomonas aeruginosa, can convert Hg2+ to Hg, which is less toxic to humans.
Probably one of the most useful and interesting examples of the use of prokaryotes for bioremediation purposes is the cleanup of oil spills. The importance of prokaryotes to petroleum bioremediation has been demonstrated in several oil spills in recent years, such as the Exxon Valdez spill in Alaska (1989) (Figure \(1\)), the Prestige oil spill in Spain (2002), the spill into the Mediterranean from a Lebanon power plant (2006,) and more recently, the BP oil spill in the Gulf of Mexico (2010). To clean up these spills, bioremediation is promoted by adding inorganic nutrients that help bacteria already present in the environment to grow. Hydrocarbon-degrading bacteria feed on the hydrocarbons in the oil droplet, breaking them into inorganic compounds. Some species, such as Alcanivorax borkumensis, produce surfactants that solubilize the oil, while other bacteria degrade the oil into carbon dioxide. In the case of oil spills in the ocean, ongoing, natural bioremediation tends to occur, inasmuch as there are oil-consuming bacteria in the ocean prior to the spill. Under ideal conditions, it has been reported that up to 80 percent of the nonvolatile components in oil can be degraded within 1 year of the spill. Researchers have genetically engineered other bacteria to consume petroleum products; indeed, the first patent application for a bioremediation application in the U.S. was for a genetically modified oil-eating bacterium.
There are a number of cost/efficiency advantages to bioremediation, which can be employed in areas that are inaccessible without excavation. For example, hydrocarbon spills (specifically, oil spills) or certain chlorinated solvents may contaminate groundwater, which can be easier to treat using bioremediation than more conventional approaches. This is typically much less expensive than excavation followed by disposal elsewhere, incineration, or other off-site treatment strategies. It also reduces or eliminates the need for “pump and treat”, a practice common at sites where hydrocarbons have contaminated clean groundwater. Using prokaryotes for bioremediation of hydrocarbons also has the advantage of breaking down contaminants at the molecular level, as opposed to simply chemically dispersing the contaminant. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/37%3A_Plant_Nutrition_and_Soils/37.05%3A_Phytoremediation/37.5.01%3A_Bioremediation.txt |
Learning Objectives
• Identify plant defense responses to herbivores
Defense Responses Against Herbivores
Herbivores, both large and small, use plants as food and actively chew them. Plants have developed a variety of strategies to discourage or kill attackers.
Mechanical Defenses
The first line of defense in plants is an intact and impenetrable barrier composed of bark and a waxy cuticle. Both protect plants against herbivores. Other adaptations against herbivores include hard shells, thorns (modified branches), and spines (modified leaves). They discourage animals by causing physical damage or by inducing rashes and allergic reactions. Some Acacia tree species have developed mutualistic relationships with ant colonies: they offer the ants shelter in their hollow thorns in exchange for the ants’ defense of the tree’s leaves.
Chemical Defenses
A plant’s exterior protection can be compromised by mechanical damage, which may provide an entry point for pathogens. If the first line of defense is breached, the plant must resort to a different set of defense mechanisms, such as toxins and enzymes. Secondary metabolites are compounds that are not directly derived from photosynthesis and are not necessary for respiration or plant growth and development.
Many metabolites are toxic and can even be lethal to animals that ingest them. Some metabolites are alkaloids, which discourage predators with noxious odors (such as the volatile oils of mint and sage) or repellent tastes (like the bitterness of quinine). Other alkaloids affect herbivores by causing either excessive stimulation (caffeine is one example) or the lethargy associated with opioids. Some compounds become toxic after ingestion; for instance, glycol cyanide in the cassava root releases cyanide only upon ingestion by the herbivore. Foxgloves produce several deadly chemicals, namely cardiac and steroidal glycosides. Ingestion can cause nausea, vomiting, hallucinations, convulsions, or death.
Timing
Mechanical wounding and predator attacks activate defense and protective mechanisms in the damaged tissue and elicit long-distancing signaling or activation of defense and protective mechanisms at sites farther from the injury location. Some defense reactions occur within minutes, while others may take several hours. In addition, long-distance signaling elicits a systemic response aimed at deterring predators. As tissue is damaged, jasmonates may promote the synthesis of compounds that are toxic to predators. Jasmonates also elicit the synthesis of volatile compounds that attract parasitoids: insects that spend their developing stages in or on another insect, eventually killing their host. The plant may activate abscission of injured tissue if it is damaged beyond repair.
Key Points
• Many plants have impenetrable barriers, such as bark and waxy cuticles, or adaptations, such as thorns and spines, to protect them from herbivores.
• If herbivores breach a plant’s barriers, the plant can respond with secondary metabolites, which are often toxic compounds, such as glycol cyanide, that may harm the herbivore.
• When attacked by a predator, damaged plant tissue releases jasmonate hormones that promote the release of volatile compounds, attracting parasitoids, which use, and eventually kill, the predators as host insects.
38.1B: Plant Defenses Against Pathogens
Plants defend against pathogens with barriers, secondary metabolites, and antimicrobial compounds.
Learning Objectives
• Identify plant defense responses to pathogens
Defense Responses Against Pathogens
Pathogens are agents of disease. These infectious microorganisms, such as fungi, bacteria, and nematodes, live off of the plant and damage its tissues. Plants have developed a variety of strategies to discourage or kill attackers.
The first line of defense in plants is an intact and impenetrable barrier composed of bark and a waxy cuticle. Both protect plants against pathogens.
A plant’s exterior protection can be compromised by mechanical damage, which may provide an entry point for pathogens. If the first line of defense is breached, the plant must resort to a different set of defense mechanisms, such as toxins and enzymes. Secondary metabolites are compounds that are not directly derived from photosynthesis and are not necessary for respiration or plant growth and development. Many metabolites are toxic and can even be lethal to animals that ingest them.
Additionally, plants have a variety of inducible defenses in the presence of pathogens. In addition to secondary metabolites, plants produce antimicrobial chemicals, antimicrobial proteins, and antimicrobial enzymes that are able to fight the pathogens. Plants can close stomata to prevent the pathogen from entering the plant. A hypersensitive response, in which the plant experiences rapid cell death to fight off the infection, can be initiated by the plant; or it may use endophyte assistance: the roots release chemicals that attract other beneficial bacteria to fight the infection.
Mechanical wounding and predator attacks activate defense and protective mechanisms in the damaged tissue and elicit long-distancing signaling or activation of defense and protective mechanisms at sites farther from the injury location. Some defense reactions occur within minutes, while others may take several hours.
Key Points
• Many plants have impenetrable barriers, such as bark and waxy cuticles, or adaptations, such as thorns and spines, to protect them from pathogens.
• If pathogens breach a plant’s barriers, the plant can respond with secondary metabolites, which are often toxic compounds, such as glycol cyanide, that may harm the pathogen.
• Plants produce antimicrobial chemicals, antimicrobial proteins, and antimicrobial enzymes that are able to fight the pathogens.
Contributions and Attributions
• Plant defense against herbivory. Provided by: Wikimedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikimedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikimedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikimedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Plant_defense_against_herbivory. License: Public Domain: No Known Copyright | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/38%3A_Plant_Defense_Responses/38.01%3A_Physical_Defenses/38.1A%3A_Plant_Defenses_Against_Herbivores.txt |
Learning Objectives
• Identify plant defense responses to herbivores
Defense Responses Against Herbivores
Herbivores, both large and small, use plants as food and actively chew them. Plants have developed a variety of strategies to discourage or kill attackers.
Mechanical Defenses
The first line of defense in plants is an intact and impenetrable barrier composed of bark and a waxy cuticle. Both protect plants against herbivores. Other adaptations against herbivores include hard shells, thorns (modified branches), and spines (modified leaves). They discourage animals by causing physical damage or by inducing rashes and allergic reactions. Some Acacia tree species have developed mutualistic relationships with ant colonies: they offer the ants shelter in their hollow thorns in exchange for the ants’ defense of the tree’s leaves.
Chemical Defenses
A plant’s exterior protection can be compromised by mechanical damage, which may provide an entry point for pathogens. If the first line of defense is breached, the plant must resort to a different set of defense mechanisms, such as toxins and enzymes. Secondary metabolites are compounds that are not directly derived from photosynthesis and are not necessary for respiration or plant growth and development.
Many metabolites are toxic and can even be lethal to animals that ingest them. Some metabolites are alkaloids, which discourage predators with noxious odors (such as the volatile oils of mint and sage) or repellent tastes (like the bitterness of quinine). Other alkaloids affect herbivores by causing either excessive stimulation (caffeine is one example) or the lethargy associated with opioids. Some compounds become toxic after ingestion; for instance, glycol cyanide in the cassava root releases cyanide only upon ingestion by the herbivore. Foxgloves produce several deadly chemicals, namely cardiac and steroidal glycosides. Ingestion can cause nausea, vomiting, hallucinations, convulsions, or death.
Timing
Mechanical wounding and predator attacks activate defense and protective mechanisms in the damaged tissue and elicit long-distancing signaling or activation of defense and protective mechanisms at sites farther from the injury location. Some defense reactions occur within minutes, while others may take several hours. In addition, long-distance signaling elicits a systemic response aimed at deterring predators. As tissue is damaged, jasmonates may promote the synthesis of compounds that are toxic to predators. Jasmonates also elicit the synthesis of volatile compounds that attract parasitoids: insects that spend their developing stages in or on another insect, eventually killing their host. The plant may activate abscission of injured tissue if it is damaged beyond repair.
Key Points
• Many plants have impenetrable barriers, such as bark and waxy cuticles, or adaptations, such as thorns and spines, to protect them from herbivores.
• If herbivores breach a plant’s barriers, the plant can respond with secondary metabolites, which are often toxic compounds, such as glycol cyanide, that may harm the herbivore.
• When attacked by a predator, damaged plant tissue releases jasmonate hormones that promote the release of volatile compounds, attracting parasitoids, which use, and eventually kill, the predators as host insects.
38.2B: Plant Defenses Against Pathogens
Plants defend against pathogens with barriers, secondary metabolites, and antimicrobial compounds.
Learning Objectives
• Identify plant defense responses to pathogens
Defense Responses Against Pathogens
Pathogens are agents of disease. These infectious microorganisms, such as fungi, bacteria, and nematodes, live off of the plant and damage its tissues. Plants have developed a variety of strategies to discourage or kill attackers.
The first line of defense in plants is an intact and impenetrable barrier composed of bark and a waxy cuticle. Both protect plants against pathogens.
A plant’s exterior protection can be compromised by mechanical damage, which may provide an entry point for pathogens. If the first line of defense is breached, the plant must resort to a different set of defense mechanisms, such as toxins and enzymes. Secondary metabolites are compounds that are not directly derived from photosynthesis and are not necessary for respiration or plant growth and development. Many metabolites are toxic and can even be lethal to animals that ingest them.
Additionally, plants have a variety of inducible defenses in the presence of pathogens. In addition to secondary metabolites, plants produce antimicrobial chemicals, antimicrobial proteins, and antimicrobial enzymes that are able to fight the pathogens. Plants can close stomata to prevent the pathogen from entering the plant. A hypersensitive response, in which the plant experiences rapid cell death to fight off the infection, can be initiated by the plant; or it may use endophyte assistance: the roots release chemicals that attract other beneficial bacteria to fight the infection.
Mechanical wounding and predator attacks activate defense and protective mechanisms in the damaged tissue and elicit long-distancing signaling or activation of defense and protective mechanisms at sites farther from the injury location. Some defense reactions occur within minutes, while others may take several hours.
Key Points
• Many plants have impenetrable barriers, such as bark and waxy cuticles, or adaptations, such as thorns and spines, to protect them from pathogens.
• If pathogens breach a plant’s barriers, the plant can respond with secondary metabolites, which are often toxic compounds, such as glycol cyanide, that may harm the pathogen.
• Plants produce antimicrobial chemicals, antimicrobial proteins, and antimicrobial enzymes that are able to fight the pathogens.
Contributions and Attributions
• Plant defense against herbivory. Provided by: Wikimedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikimedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikimedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikimedia. Located at: en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No Known Copyright
• Plant defense against herbivory. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Plant_defense_against_herbivory. License: Public Domain: No Known Copyright | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/38%3A_Plant_Defense_Responses/38.02%3A_Chemical_Defenses/38.2A%3A_Plant_Defenses_Against_Herbivores.txt |
Learning Objectives
• Identify several signaling molecules beyond the five major plant hormones and describe their effects.
• Distinguish between the hypersensitive response and systemic acquired response.
• Explain the mechanisms by which signaling compounds aid in plant defense against pathogens and herbivores.
Recent research has discovered a number of compounds that also influence plant development. Their roles are less understood than the effects of the major hormones described so far.
Brassinosteroids
Brassinosteroids (Figure \(1\)) are synthesized primarily in young tissues are important to many developmental and physiological processes. In fact, many sources considere them the sixth major plant hormones. Unlike the hormones discussed previously, brassinosteroids do not travel far from their site of synthesis. Signals between these compounds and other hormones, notably auxin and GAs, amplifies their physiological effect. Apical dominance, seed germination, gravitropism, lateral root formation, differentiation of cells in the vascular tissue, and resistance to freezing are all positively influenced by brassinosteroids. Root growth and fruit dropping are inhibited by steroids.
Systemin
Systemin, named for the fact that it is distributed systemically (everywhere) in the plant body upon production, is a short polypeptide that activates plant responses to wounds from herbivores (animals that feed on plant parts). It causes the plant to produce jasmonic acid (see below).
Jasmonates
Jasmonates play a major role in defense responses to herbivory (Figure \(2\)). Their levels increase when a plant is wounded by an herbivore, resulting in an increase in toxic secondary metabolites. For example, jasmonic acid (Figure \(3\)) also induces transcription of protease inhibitors. Protease inhibitors both taste bad and prevent breakdown of proteins in the herbivore’s gut, thus making the insect sick and deterring further herbivory. Jasmonates also contribute to the production of volatile compounds that attract natural enemies of herbivores. Chewing of tomato plants by caterpillars leads to an increase in jasmonic acid levels, which in turn triggers the release of volatile compounds that attract predators of the pest. Jasmonates also elicit the synthesis of volatile compounds that attract parasitoids, which are insects that spend their developing stages in or on another insect, and eventually kill their host.
Jasmonates also work with systemin to mediate responses to drought, damage by ground-level ozone, and ultraviolet light.
Salicylic Acid
Salicylic acid resembles aspirin (Figure \(4\)) and is important for plant defense. It initiates the a systemic (whole body) response called the systemic acquired response (SAR) as a response to infection by parasites or pathogens. When a parasite or pathogen infects a cell, there is an specific, localized response called the hypersensitive response (HR). Following this very localized response, the plant initiates a systemic (whole body) response called the systemic acquired response (SAR). Salicylic acid is produced and converted to methyl salicylate (Figure \(4\)) inducing the SAR in response to the HR. The SAR activates transcription of general “pathogenesis-resistance” genes, which are not pathogen-specific (unlike in the hypersensitive response), but serve as general defense against pathogenic infection. The SAR is slower than the hypersensitive response, and also differs in that it is systemic instead of localized to the site of the infection.
Similar to jasmonic acid, salicylic acid can mediates defense against insect herbivores. It is directly toxic to some herbivores. Additionally, in response to herbivory, salicylic acid can be converted to methyl salicylate, which is released as a gas. This volatile compound can attract natural predators and parasites of the herbivores.
Some plants, such as skunk cabbage (Figure \(5\)) and elephant yam, are adapted to flower while snow still covers the ground. Salicylic acid mediates their ability to produce heat to melt the snow around them. Such plants are thus called thermogenic ("heat producing").
Oligosaccharins
Oligosaccharins are short chains of simple sugars that play a role in plant defense against bacterial and fungal infections. They act locally at the site of injury, and can also be transported to other tissues.
Strigolactones
Strigolactones (Figure \(6\)) promote seed germination in some species and inhibit lateral apical development in the absence of auxins. Strigolactones also play a role in the establishment of mycorrhizae, a mutualistic association of plant roots and fungi.
Florigen
Florigen is a systemic signal that initiates flowering. It is also involved in the formation of storage organs and contributes to plant architecture. It is synthesized in leaves and transported to the shoot apical meristem (SAM) where it promotes flowering in response to daylength cues. At the molecular level, florigen is represented as a protein product encoded by the FLOWERING LOCUS T (FT) gene, which is highly conserved (occurs/has a similar genetic sequence in) across flowering plants.
Florigen is considered one of the important targets for crop improvement. Regulation of flowering time is an important target for plant breeding because the control of flowering to a favorable time provides successful grain production in a given cropping area. Flowering at unfavorable seasons causes loss of yield due to insufficient growth of photosynthetic organs or poor fertility due to heat or cold stress during reproduction. Thus, understanding florigen function can contribute to novel breeding techniques in crops to produce cultivars that can start their reproductive stage at optimal seasons.
Supplemental Reading
Filgueiras, C. C., Martins, A. D., Pereira, R. V., & Willett, D. S. (2019). The Ecology of Salicylic Acid Signaling: Primary, Secondary and Tertiary Effects with Applications in Agriculture. International journal of molecular sciences, 20 (23), 5851. https://doi.org/10.3390/ijms20235851
Attributions
Curated and authored by Melissa Ha from the following sources: | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/38%3A_Plant_Defense_Responses/38.04%3A_Systemic_Responses_to_Invaders/38.4.01%3A_Jasmonates.txt |
Learning Objectives
• Compare the ways plants respond to light
Plant Responses to Light
Plants have a number of sophisticated uses for light that go far beyond their ability to perform photosynthesis. Plants can differentiate and develop in response to light (known as photomorphogenesis), which allows plants to optimize their use of light and space. Plants use light to track time, which is known as photoperiodism. They can tell the time of day and time of year by sensing and using various wavelengths of sunlight. Light can also elicit a directional response in plants that allows them to grow toward, or even away from, light; this is known as phototropism.
The sensing of light in the environment is important to plants; it can be crucial for competition and survival. The response of plants to light is mediated by different photoreceptors: a protein covalently-bonded to a light-absorbing pigment called a chromophore; together, called a chromoprotein. The chromophore of the photoreceptor absorbs light of specific wavelengths, causing structural changes in the photoreceptor protein. The structural changes then elicit a cascade of signaling throughout the plant.
The red, far-red, and violet-blue regions of the visible light spectrum trigger structural development in plants. Sensory photoreceptors absorb light in these particular regions of the visible light spectrum because of the quality of light available in the daylight spectrum. In terrestrial habitats, light absorption by chlorophylls peaks in the blue and red regions of the spectrum. As light filters through the canopy and the blue and red wavelengths are absorbed, the spectrum shifts to the far-red end, shifting the plant community to those plants better adapted to respond to far-red light. Blue-light receptors allow plants to gauge the direction and abundance of sunlight, which is rich in blue–green emissions. Water absorbs red light, which makes the detection of blue light essential for algae and aquatic plants.
Key Points
• Plants grow and differentiate to optimize their space, using light in a process known as photomorphogenesis.
• Plants grow and move toward or away from light depending on their needs; this process is known as phototropism.
• Photoperiodism is illustrated by how plants flower and grow at certain times of the day or year through the use of photoreceptors that sense the wavelengths of sunlight available during the day (versus night) and throughout the seasons.
• The various wavelengths of light, red/far-red or blue regions of the visible light spectrum, trigger structural responses in plants suited for responding to those wavelengths.
Key Terms
• photoreceptor: a specialized protein that is able to detect and react to light
• photoperiodism: the growth, development and other responses of plants and animals according to the length of day and/or night
• photomorphogenesis: the regulatory effect of light on the growth, development and differentiation of plant cells, tissues and organs
• phototropism: the movement of a plant toward or away from light
39.1.02: Photoperiodism
Learning Objectives
• Describe the mechanism of photoperiodism with respect to flowering.
• Distinguish among short-day, long-day, and day-neutral plants.
• Define circadian rhythms and provide examples in plants.
Detection of seasonal changes is crucial to plant survival. Although temperature and light intensity influence plant growth, they are not reliable indicators of season because they may vary from one year to the next. Day length is a better indicator of the time of year. Many angiosperms flower at about the same time every year. This occurs even though they may have started growing at different times. Their flowering is a response to the changing length of day and night as the season progresses. It helps promote cross pollination. The biological response to the timing and duration of day and night is called photoperiodism.
The Phytochrome System and the Red/Far-Red Response
Plants use the phytochrome system to sense the change of season, which can control flowering. The phytochromes are a family of photoreceptors. They are chromoproteins with a linear tetrapyrrole chromophore (a molecule that absorbs light), similar to the ringed tetrapyrrole light-absorbing head group of chlorophyll. A phytochrome is a homodimer: two identical protein molecules, each conjugated to a light-absorbing molecule (compare to rhodopsin). Plants make 5 phytochromes: PhyA, PhyB, as well as C, D, and E. There is some redundancy in function of the different phytochromes, but there also seem to be functions that are unique to one or another. The phytochromes also differ in their absorption spectrum; that is, which wavelengths (e.g., red vs. far-red) they absorb best. Phytochromes have two photo-interconvertible forms: Pr and Pfr. Pr absorbs red light (~667 nm) and is immediately converted to Pfr. Pfr absorbs far-red light (~730 nm) and is quickly converted back to Pr. Absorption of red or far-red light causes a massive change to the shape of the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. Pfr is the physiologically active form of the protein; therefore, exposure to red light yields physiological activity. Exposure to far-red light inhibits phytochrome activity. Together, the two forms represent the phytochrome system (Figure \(1\)).
The phytochrome system acts as a biological light switch. It monitors the level, intensity, duration, and color of environmental light. The effect of red light is reversible by immediately shining far-red light on the sample, which converts the chromoprotein to the inactive Pr form. Additionally, Pfr can slowly revert to Pr in the dark, or break down over time. In all instances, the physiological response induced by red light is reversed. The active form of phytochrome (Pfr) can directly activate other molecules in the cytoplasm, or it can be transported to the nucleus, where it directly activates or represses specific gene expression.
Unfiltered sunlight is rich in red light but deficient in far-red light. Therefore, at dawn, all the phytochrome molecules in a leaf quickly convert to the active Pfr form, and remain in that form until sunset. In the dark, the Pfr form takes hours to slowly revert back to the Pr form. If the night is long (as in winter), all of the Pfr form reverts. If the night is short (as in summer), a considerable amount of Pfr may remain at sunrise. By sensing the Pr/Pfr ratio at dawn, a plant can determine the length of the day/night cycle. In addition, leaves retain that information for several days, allowing a comparison between the length of the previous night and the preceding several nights. Shorter nights indicate springtime to the plant; when the nights become longer, autumn is approaching. This information, along with sensing temperature and water availability, allows plants to determine the time of the year and adjust their physiology accordingly.
In 1920 two employees of the U. S. Department of Agriculture, W. W. Garner and H. A. Allard, discovered a mutation in tobacco - a variety called Maryland Mammoth - that prevented the plant from flowering in the summer as normal tobacco plants do. Maryland Mammoth would not bloom until late December. Experimenting with artificial lighting in winter and artificial darkening in summer, they found that Maryland Mammoth was affected by photoperiod. Because it would flower only when exposed to short periods of light, they called it a short-day plant. Examples of other short-day plants include chrysanthemums, rice (Oryza sativa), poinsettias, morning glory (Pharbitis nil), and cocklebur (Xanthium).
Experiments with the cocklebur have shown that the term short-day is something of a misnomer; what the cocklebur needs is a sufficiently long night (Figure \(2\)). Short-day (long-night) plants flower in the late summer and early fall, when nights exceed a critical length (often eight or fewer hours). In short-day plants, the active form of phytochrome (Pfr) suppresses flowering. During long periods of darkness (long nights), Pfr is converted to Pr. With Pfr no longer present, flowering is not suppressed, and short-day plants flower. If a flash of light interrupts the dark period, Pr is converted back to Pfr, and flowering is suppressed.
Long-day (short-night) plants flower during the spring, when darkness is less than a critical length (often eight to 15 hours). Examples include spinach, Arabidopsis, sugar beet, and the radish flower.
Flowering in day-neutral plants, such as the tomato, is not regulated by photoperiod.
Photoperiodism also explains why some plant species can be grown only in a certain latitude. Spinach, a long-day plant, cannot flower in the tropics because the days never get long enough (14 hours). Ragweed, a short-day plant, fails to thrive in northern Maine because by the time the days become short enough to initiate flowering, a killing frost in apt to occur before reproduction and the formation of seeds is completed.
Some plants do not neatly fit into the categories of short day, long day, or day neutral. In 1941, Marie Taylor Clark found that flowering in scarlet sage did not flower under daylengths longer than 16 hours, suggesting it was a short-day plant; however, days that were too short (6 hours) slowed flower development. Flower development was optimal with daylengths of 10 hours.
The leaves produce a chemical signal called florigen that is transmitted to the apical meristems to start their conversion into floral meristems. The chemical nature of florigen has been sought for decades. The most recent evidence suggests that at least one component is the protein encoded by the gene FLOWERING LOCUS T (FT). Due to florigen signaling, the entire plant will bloom even if only a part of one leaf is exposed to the correct photoperiod (Figure \(3\)).
Career Connection: Horticulturist
The word “horticulturist” comes from the Latin words for garden (hortus) and culture (cultura). This career has been revolutionized by progress made in the understanding of plant responses to environmental stimuli. Growers of crops, fruit, vegetables, and flowers were previously constrained by having to time their sowing and harvesting according to the season. Now, horticulturists can manipulate plants to increase leaf, flower, or fruit production by understanding how environmental factors affect plant growth and development.
Greenhouse management is an essential component of a horticulturist’s education. To lengthen the night, plants are covered with a blackout shade cloth. Long-day plants are irradiated with red light in winter to promote early flowering. For example, fluorescent (cool white) light high in blue wavelengths encourages leafy growth and is excellent for starting seedlings. Incandescent lamps (standard light bulbs) are rich in red light, and promote flowering in some plants. The timing of fruit ripening can be increased or delayed by applying plant hormones. Recently, considerable progress has been made in the development of plant breeds that are suited to different climates and resistant to pests and transportation damage. Both crop yield and quality have increased as a result of practical applications of the knowledge of plant responses to external stimuli and hormones.
Horticulturists find employment in private and governmental laboratories, greenhouses, botanical gardens, and in the production or research fields (Figure \(4\)). They improve crops by applying their knowledge of genetics and plant physiology. To prepare for a horticulture career, students take classes in botany, plant physiology, plant pathology, landscape design, and plant breeding. To complement these traditional courses, horticulture majors add studies in economics, business, computer science, and communications.
Circadian Rhythms
Circadian rhythms are changes based on a 24-hour cycle. For example, flowers might open every morning and close every evening or vice versa. In Oxalis and silk tree (Albizia julibrissin), leaflets expand during the day and retract at night. Circadian rhythms may also involve physiological processes like photosynthetic rate or the the production of floral scent compounds.
Under constant conditions, circadian rhythms may drift out of phase with the environment (figure \(5\)). However, when exposed to environmental changes (e.g., alternating day and night), the rhythms become entrained; that is, they now cycle synchronized with the cycle of day and night with a period of exactly 24 hours. Internal circadian clocks also adjust to changing photoperiods. Suppose a plant that flowers throughout the spring opens its flowers every morning. The sun rises earlier in the late spring compared to the early spring (photoperiod increases in late spring). As time passes and the plant detects the changing photoperiod (technically, plants measure the length of the night rather than daylength; see above), the circadian clock would adjust such that its flowers opened earlier. In Arabidopsis, the entrainment of circadian rhythms requires that light is detected by phytochromes (absorb red light) and cryptochromes (absorb blue light).
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/39%3A_Sensory_Systems_in_Plants/39.01%3A_Responses_to_Light/39.1.01%3A_Plant_Responses_to_Light.txt |
Learning Objectives
• Explain the response of the phytochrome system to red/far-red light
The phytochromes are a family of chromoproteins with a linear tetrapyrrole chromophore, similar to the ringed tetrapyrrole light-absorbing head group of chlorophyll. Phytochromes have two photo-interconvertible forms: Pr and Pfr. Pr absorbs red light (~667 nm) and is immediately converted to Pfr. Pfr absorbs far-red light (~730 nm) and is quickly converted back to Pr. Absorption of red or far-red light causes a massive change to the shape of the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. Pfr is the physiologically-active form of the protein; exposure to red light yields physiological activity in the plant. Exposure to far-red light converts the Pfr to the inactive Pr form, inhibiting phytochrome activity. Together, the two forms represent the phytochrome system.
The phytochrome system acts as a biological light switch. It monitors the level, intensity, duration, and color of environmental light. The effect of red light is reversible by immediately shining far-red light on the sample, which converts the chromoprotein to the inactive Pr form. Additionally, Pfr can slowly revert to Pr in the dark or break down over time. In all instances, the physiological response induced by red light is reversed. The active form of phytochrome (Pfr) can directly activate other molecules in the cytoplasm, or it can be trafficked to the nucleus, where it directly activates or represses specific gene expression.
The Phytochrome System and Growth
Plants use the phytochrome system to grow away from shade and toward light. Unfiltered, full sunlight contains much more red light than far-red light. Any plant in the shade of another plant will be exposed to red-depleted, far-red-enriched light because the other plant has absorbed most of the other red light. The exposure to red light converts phytochrome in the shaded leaves to the Pr (inactive) form, which slows growth. The leaves in full sunlight are exposed to red light and have activated Pfr, which induces growth toward sunlit areas. Because competition for light is so fierce in a dense plant community, those plants who could grow toward light the fastest and most efficiently became the most successful.
The Phytochrome System in Seeds
In seeds, the phytochrome system is used to determine the presence or absence of light, rather than the quality. This is especially important in species with very small seeds and, therefore, food reserves. For example, if lettuce seedlings germinated a centimeter under the soil surface, the seedling would exhaust its food resources and die before reaching the surface. A seed will only germinate if exposed to light at the surface of the soil, causing Pr to be converted to Pfr, signaling the start of germination. In the dark, phytochrome is in the inactive Pr form so the seed will not germinate.
Photoperiodism
Plants also use the phytochrome system to adjust growth according to the seasons. Photoperiodism is a biological response to the timing and duration of dark and light periods. Since unfiltered sunlight is rich in red light, but deficient in far-red light, at dawn, all the phytochrome molecules in a leaf convert to the active Pfr form and remain in that form until sunset. Since Pfr reverts to Pr during darkness, there will be no Pfr remaining at sunrise if the night is long (winter) and some Pfr remaining if the night is short (summer). The amount of Pfr present stimulates flowering, setting of winter buds, and vegetative growth according to the seasons.
In addition, the phytochrome system enables plants to compare the length of dark periods over several days. Shortening nights indicate springtime to the plant; lengthening nights indicate autumn. This information, along with sensing temperature and water availability, allows plants to determine the time of the year and adjust their physiology accordingly. Short-day (long-night) plants use this information to flower in the late summer and early fall when nights exceed a critical length (often eight or fewer hours). Long-day (short-night) plants flower during the spring when darkness is less than a critical length (often 8 to 15 hours). However, day-neutral plants do not regulate flowering by day length. Not all plants use the phyotochrome system to adjust their physiological responses to the seasons.
Key Points
• Exposure to red light converts the chromoprotein to the functional, active form (Pfr), while darkness or exposure to far-red light converts the chromophore to the inactive form (Pr).
• Plants grow toward sunlight because the red light from the sun converts the chromoprotein into the active form (Pfr), which triggers plant growth; plants in shade slow growth because the inactive form (Pr) is produced.
• If seeds sense light using the phytochrome system, they will germinate.
• Plants regulate photoperiodism by measuring the Pfr/Pr ratio at dawn, which then stimulates physiological processes such as flowering, setting winter buds, and vegetative growth.
Key Terms
• phytochrome: any of a class of pigments that control most photomorphogenic responses in higher plants
• chromophore: the group of atoms in a molecule in which the electronic transition responsible for a given spectral band is located
• photoperiodism: the growth, development and other responses of plants and animals according to the length of day and/or night | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/39%3A_Sensory_Systems_in_Plants/39.01%3A_Responses_to_Light/39.1B%3A_The_Phytochrome_System_and_Red_Light_Response.txt |
Learning Objectives
• Differentiate among blue light responses of plants
Phototropism is the directional bending of a plant toward or away from a light source of blue wavelengths of light. Positive phototropism is growth toward a light source, while negative phototropism (also called skototropism) is growth away from light. Several proteins use blue light to control various physiological processes in the plant.
Phototropins and Physiological Responses
Phototropins are protein-based receptors responsible for mediating the phototropic response in plants. Like all plant photoreceptors, phototropins consist of a protein portion and a light-absorbing portion, called the chromophore, which senses blue wavelengths of light. Phototropins belong to a class of proteins called flavoproteins because the chromophore is a covalently-bound molecule of flavin.
Phototropins control other physiological responses including leaf opening and closing, chloroplast movement, and the opening of stomata. However, of all responses controlled by phototropins, phototropism has been studied the longest and is the best understood.
Phototropism and Auxin
In 1880, Charles Darwin and his son Francis first described phototropism as the bending of seedlings toward light. Darwin observed that light was perceived by the the apical meristem (tip of the plant), but that the plant bent in response in a different part of the plant. The Darwins concluded that the signal had to travel from the apical meristem to the base of the plant, where it bent.
In 1913, Peter Boysen-Jensen conducted an experiment that demonstrated that a chemical signal produced in the plant tip was responsible for the plant’s bending response at the base. He cut off the tip of a seedling, covered the cut section with a permeable layer of gelatin, and then replaced the tip. The seedling bent toward the light when illuminated even though the layer of gelatin was present. However, when impermeable mica flakes were inserted between the tip and the cut base, the seedling did not bend.
A refinement of Boysen-Jensen’s experiment showed that the signal traveled on the shaded side of the seedling. When the mica plate was inserted on the illuminated side, the plant still bent toward the light. Therefore, the chemical signal from the sunlight, which is blue wavelengths of light, was a growth stimulant; the phototropic response involved faster cell elongation on the shaded side than on the illuminated side, causing the plant to bend. We now know that as light passes through a plant stem, it is diffracted and generates phototropin activation across the stem. Most activation occurs on the lit side, causing the plant hormones indole acetic acid (IAA) or auxin to accumulate on the shaded side. Stem cells elongate under the influence of IAA.
Cryptochromes
Cryptochromes are another class of blue-light absorbing photoreceptors. Their chromophores also contain a flavin-based chromophore. Cryptochromes set the plant’s circadian rhythm (the 24-hour activity cycle) using blue light receptors. There is some evidence that cryptochromes work by sensing light-dependent redox reactions and that, together with phototropins, they mediate the phototropic response.
Key Points
• In addition to phototropism, phototropins sense blue light to control leaf opening and closing, chloroplast movement, and the opening of stomata.
• When phototropins are activated by blue light, the hormone auxin accumulates on the shaded side of the plant, triggering elongation of stem cells and phototropism.
• Cryptochromes sense blue light-dependent redox reactions to control the circadian rhythm of plants.
Key Terms
• skototropism: growth or movement away from light
• phototropin: any of a class of photoreceptor flavoproteins that mediate phototropism in higher plants
• auxin: a class of plant growth hormones that is responsible for elongation in phototropism and gravitropism and for other growth processes in the plant life cycle
• cryptochrome: any of several light-sensitive flavoproteins, in the protoreceptors of plants, that regulate germination, elongation, and photoperiodism | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/39%3A_Sensory_Systems_in_Plants/39.01%3A_Responses_to_Light/39.1C%3A_Blue_Light_Response.txt |
Learning Objectives
• Describe the role of amyloplasts in gravitropism
Whether or not they germinate in the light or in total darkness, shoots usually sprout up from the ground, while roots grow downward into the ground. A plant laid on its side in the dark will send shoots upward when given enough time. Gravitropism ensures that roots grow into the soil and that shoots grow toward sunlight. Growth of the shoot apical tip upward is called negative gravitropism, whereas growth of the roots downward is called positive gravitropism.
Time-lapse of pea shoot and root growth: Time-lapse of a pea plant growing from seed, showing both the shoot and root system. The roots grown downward in the direction of gravity, which is positive gravitropism, and the shoot grows upward away from gravity, which is negative gravitropism.
The reason plants know which way to grow in response to gravity is due to amyloplasts in the plants. Amyloplasts (also known as statoliths ) are specialized plastids that contain starch granules and settle downward in response to gravity. Amyloplasts are found in shoots and in specialized cells of the root cap. When a plant is tilted, the statoliths drop to the new bottom cell wall. A few hours later, the shoot or root will show growth in the new vertical direction.
The mechanism that mediates gravitropism is reasonably well understood. When amyloplasts settle to the bottom of the gravity-sensing cells in the root or shoot, they physically contact the endoplasmic reticulum (ER). This causes the release of calcium ions from inside the ER. This calcium signaling in the cells causes polar transport of the plant hormone indole acetic acid (IAA) to the bottom of the cell. In roots, a high concentration of IAA inhibits cell elongation. The effect slows growth on the lower side of the root while cells develop normally on the upper side. IAA has the opposite effect in shoots, where a higher concentration at the lower side of the shoot stimulates cell expansion and causes the shoot to grow up. After the shoot or root begin to grow vertically, the amyloplasts return to their normal position. Other hypotheses, which involve the entire cell in the gravitropism effect, have been proposed to explain why some mutants that lack amyloplasts may still exhibit a weak gravitropic response.
Key Points
• Positive gravitropism occurs when roots grow into soil because they grow in the direction of gravity while negative gravitropism occurs when shoots grow up toward sunlight in the opposite direction of gravity.
• Amyloplasts settle at the bottom of the cells of the shoots and roots in response to gravity, causing calcium signaling and the release of indole acetic acid.
• Indole acetic acid inhibits cell elongation in the lower side of roots, but stimulates cell expansion in shoots, which causes shoots to grow upward.
Key Terms
• amyloplast: a non-pigmented organelle found in some plant cells that is responsible for the synthesis and storage of starch granules through the polymerization of glucose
• statolith: a specialized form of amyloplast involved in graviperception by plant roots and most invertebrates
• gravitropism: a plant’s ability to change its growth in response to gravity
39.3G: Plant Responses to Wind and Touch
Learning Objectives
• Compare the ways plants respond to directional and non-directional stimuli
The shoot of a pea plant wraps around a trellis while a tree grows on an angle in response to strong prevailing winds. These are examples of how plants respond to touch or wind.
The movement of a plant subjected to constant directional pressure is called thigmotropism, from the Greek words thigma meaning “touch,” and tropism, implying “direction.” Tendrils are one example of this. A tendril is a specialized stem, leaf, or petiole with a threadlike shape that is used by climbing plants for support.The meristematic region of tendrils is very touch sensitive; light touch will evoke a quick coiling response. Cells in contact with a support surface contract, whereas cells on the opposite side of the support expand. Application of jasmonic acid is sufficient to trigger tendril coiling without a mechanical stimulus.
A thigmonastic response is a touch response independent of the direction of stimulus. In the Venus flytrap, two modified leaves are joined at a hinge and lined with thin, fork-like tines along the outer edges. Tiny hairs are located inside the trap. When an insect brushes against these trigger hairs, touching two or more of them in succession, the leaves close quickly, trapping the prey. Glands on the leaf surface secrete enzymes that slowly digest the insect. The released nutrients are absorbed by the leaves, which reopen for the next meal.
Thigmomorphogenesis is a slow developmental change in the shape of a plant subjected to continuous mechanical stress. When trees bend in the wind, for example, growth is usually stunted and the trunk thickens. Strengthening tissue, especially xylem, is produced to add stiffness to resist the wind’s force. Researchers hypothesize that mechanical strain from wind, rain, or movement by other living things induces growth and differentiation to strengthen the tissues. Ethylene and jasmonate are likely involved in thigmomorphogenesis.
Key Points
• When subjected to constant directional pressure, such as a trellis, plants move to grow around the object providing the pressure; this process is known as thigmotropism.
• Thigmonastic responses include opening and closing leaves, petals, or other parts of the plant as a reaction to touch.
• Through thigmomorphogenesis, plants change their growth in response to repeated mechanical stress from wind, rain, or other living things.
Key Terms
• thigmotropism: plant growth or motion in response to touch
• thigmomorphogenesis: the response by plants to mechanical sensation (touch) by altering their growth patterns
• thigmonastic response: a touch response independent of the direction of stimulus | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/39%3A_Sensory_Systems_in_Plants/39.02%3A_Responses_to_Gravity/39.2D%3A_Plant_Responses_to_Gravity.txt |
Learning Objectives
• Identify the locations of synthesis, transport, and actions of abscisic acid.
• Describe how ABA interacts with other plant hormones.
The plant hormone abscisic acid (ABA) was was once thought to be responsible for abscission; however, this is now known to be incorrect. Instead, ABA accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures, or shortened day lengths. Unlike animals, plants cannot flee from potentially harmful conditions like drought, freezing, exposure to salt water or salinated soil, and ABA plays in mediating adaptations of the plant to stress. Abscisic acid (Figure \(1\)) resembles the carotenoid zeaxanthin (Figure \(2\)), from which it is ultimately synthesized. It is produced in mature leaves and roots and transported through the vascular tissue.
Maintaining Dormancy
Seed Maturation and Inhibition of Germination
Seeds are not only important agents of reproduction and dispersal, but they are also essential to the survival of annual and biennial plants. These angiosperms die after flowering and seed formation is complete. Abscisic acid is essential for seed maturation and also enforces a period of seed dormancy, by blocking germination and promoting the synthesis of storage proteins. It is important the seeds not germinate prematurely during unseasonably mild conditions prior to the onset of winter or a dry season. As the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring. As discussed in the Environmental Responses chapter, other environmental cues such as exposure to a cold period, light, or water are often also needed to for germination to occur.
Interestingly, mangrove species with viviparous germination, meaning that seeds germinate while still attached to the parent plant have reduced levels of ABA during embryo formation, providing further evidence of ABA's role in maintain seed dormancy (Farnsworth and Farrant 1998, Am J. Bot.). These mangroves are adapted to drop germinated seeds into surrounding water to be dispersed (Figure \(3\)).
Bud Dormancy
Another effect of ABA is to promote the development of winter buds; it mediates the conversion of the apical meristem into a dormant bud. The newly developing leaves growing above the meristem become converted into stiff bud scales that wrap the meristem closely and will protect it from mechanical damage and drying out during the winter. Abscisic acid in the bud also acts to enforce dormancy so if an unseasonably warm spell occurs before winter is over, the buds will not sprout prematurely. Only after a prolonged period of cold or the lengthening days of spring (photoperiodism) will bud dormancy be lifted.
Response to Water Stress
Stomatal Closure
Abscisic acid also regulates the short-term drought response. Recall that stomata are pores in the leaf and are surrounded by a pair of guard cells. Much of the water taken up by a plant is lost as water vapor exists stomata. Low soil moisture causes an increase in ABA, which causes stomata to close, reducing water loss. Note that stomatal closure also prevents exchange of oxygen and carbon dioxide, which is necessary for efficient photosynthesis (see Photorespiration and Phytosynthetic Pathways). The response to abscisic acid occurs even if blue light is present; that is, signaling from drought via ABA overrides the signaling from blue light to open stomata. See Transport for more details about stomatal opening and closure.
Cellular Protection from Dehydration
Abscisic acid turns on the expression of genes encoding proteins that protect cells - in seeds as well as in vegetative tissues - from damage when they become dehydrated.
Interactions with Other Hormones
At a cellular level, abscisic acid inhibits both cell division and cell expansion. It often opposes the growth-inducing effects of auxin and gibberellic acid. For example, abscisic acid prevents stem elongation probably by its inhibitory effect on gibberellic acid. In maintaining apical dominance, however, ABA synergizes with auxin. Abscisic acid moves up from the roots to the stem (opposite the flow of auxin) and suppresses the development of axillary buds. The result is inhibition of branching (maintaining apical dominance).
Attributions
Curated and authored by Melissa Ha from the following sources: | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/39%3A_Sensory_Systems_in_Plants/39.04%3A_Responses_to_Water_and_Temperature/39.4.01%3A_Abscisic_Acid.txt |
Learning Objectives
• Explain the mechanism of polar auxin transport.
• Identify locations of synthesis and actions of auxin.
• Define apical dominance and explain the role of auxin in maintaining it.
• Describe the commercial applications of auxin.
• Interpret and predict outcomes of experiments that demonstrate the action of auxin.
The term auxin is derived from the Greek word auxein, which means "to grow." While many synthetic auxins are used as herbicides, indole-3-acetic acid (IAA) is the only naturally occurring auxin that shows physiological activity (Figure \(1\)). Auxin is synthesized in apical meristems, young leaves, and developing seeds.
The Discovery of Auxin
Recall from the Tropisms section that the Boysen-Jensen experiment showed a chemical signal must be downward from the tip of the coleoptile tip along the shaded side, resulting in phototropism. Went extracted the chemical signal involved in the Boysen-Jensen experiment. He removed the tips of several coleoptiles of oat, Avena sativa, seedlings. He placed these on a block of agar for several hours. At the end of this time, the agar block itself was able to initiate resumption of growth of the decapitated coleoptile. The growth was vertical because the agar block was placed completely across the stump of the coleoptile and no light reached the plant from the side (Figure \(2\)). The unknown substance that had diffused from the agar block was named auxin. The amount of auxin in coleoptile tips was far too small to be purified and analyzed chemically. Therefore, a search was made for other sources of auxin activity.
This search was aided by a technique called the Avena test developed by Went for determining the relative amount of auxin activity in a preparation. The material to be assayed is incorporated into an agar block, and the block is placed on one edge of a decapitated Avena coleoptile. As the auxin diffuses into that side of the coleoptile, it stimulates cell elongation and the coleoptile bends away from the block (Figure \(3\)). The degree of curvature, measured after 1.5 hours in the dark, is proportional to the amount of auxin activity (e.g., number of coleoptile tips used). The use of living tissue to determine the amount of a substance, such as in the Avena test, is called a bioassay.
The Avena test soon revealed that substances with auxin activity occur widely in nature. One of the most potent was first isolated from human urine. It was indole-3-acetic acid (IAA) and turned out to be the auxin actually used by plants.
Auxin Transport
Auxin moves through the plant by two mechanisms, called polar and nonpolar transport.
Polar Transport
In contrast to the other major plant hormones, auxins can be transported in a specific direction (polar transport) through parenchyma cells. The cytoplasms of parenchyma cells are neutral (pH = 7), but the region outside the plasma membranes of adjacent cells (the apoplast) is acidic (pH = 5). When auxin is in the cytoplasm, it releases a proton and becomes an anion (IAA-). It cannot pass through hydrophobic portion of the plasma membrane as an anion, but it does pass through special auxin efflux transporters called PIN proteins. Eight different types of these transmembrane proteins have been identified so far. When IAA- enters the acidic environment of the apoplast, it is protonated, becoming IAAH. This uncharged molecule can then pass through the plasma membrane of adjacent cells through diffusion or via influx transporters. Once it enters the cytoplasm, it loses its proton, becoming IAA- again. PIN proteins can be unevenly distributed around the cell (for example, only occurring on the bottom of the cell), which directs the flow of auxin (Figure \(4\)).
Nonpolar Transport
Auxins can also be transported nondirectionally (nonpolar transport) through the phloem. It passes in the assimilate that translocates through the phloem from where it is synthesized (its "source", usually the shoot) to a "sink" (e.g., the root).
Actions of Auxin
Tropisms
Auxins are the main hormones responsible for phototropism and gravitropism. The auxin gradients that are required for these tropisms are facilitated by the movement of PIN proteins and the polar transport of auxin in response to environmental stimuli (light or gravity). Note that higher auxin concentration on one side of the stem typically causes that side of the stem to elongate; however, the effect is opposite in roots with higher auxin concentration inhibiting elongation (Figure \(5\)).
Growth and Development
Embryo Development
Auxins play a role in embryo development. From the very first mitotic division of the zygote, gradients of auxin guide the patterning of the embryo into the parts that will become the organs of the plant, including the shoot apex, primary leaves, cotyledon(s), stem, and root.
Vascular Tissue Differentiation
They also control cell differentiation of vascular tissue.
Leaf Development and Arrangement
The formation of new leaves in the apical meristem is initiated by the accumulation of auxin. Already-developing leaves deplete the surrounding cells of auxin so that the new leaves do not form too close to them. In this way, the characteristic pattern of leaves in the plant is established. Auxin also controls the precise patterning of the epidermal cells of the developing leaf.
Root Initiation and Development
The localized accumulation of auxin in epidermal cells of the root initiates the formation of lateral or secondary roots. Auxin also stimulates the formation of adventitious roots in many species. Adventitious roots grow from stems or leaves rather than from the regular root system of the plant. Once a root is formed, a gradient of auxin concentration develops highest at the tip promoting the production of new cells at the meristem, and lowest in the region of differentiation, thus promoting the elongation and differentiation of root cells. The drop in auxin activity in the regions of elongation and differentiation is mediated by cytokinin — an auxin antagonist.
Shade Avoidance
Auxins stimulate cell elongation parts of the plants that have access to light as part of the shade-avoidance response (see Etiolation and Shade Avoidance).
Interactions with Other Growth-Regulating Hormones
Auxin is required for the function of other growth-regulating hormones such as cytokinins; cytokinins promote cell division, but only in the presence of auxin.
Apical Dominance
Apical dominance—the inhibition of axillary bud (lateral bud) formation—is triggered by downward transport of auxins produced in the apical meristem. Many plants grow primarily at a single apical meristem and have limited axillary branches (Figure \(6\)). Growth of the shoot apical meristem (terminal shoot) usually inhibits the development of the lateral buds on the stem beneath. If the shoot apical meristem of a plant is removed, the inhibition is lifted, and axillary buds begin growth. However, if the apical meristem is removed and IAA applied to the stump, inhibition of the axillary buds is maintained (Figure \(7\)). Gardeners exploit this principle by pruning the terminal shoot of ornamental shrubs, etc. The release of apical dominance enables lateral branches to develop and the plant becomes bushier. The process usually must be repeated because one or two laterals will eventually outstrip the others and reimpose apical dominance.
The common white potato also illustrates the principle of apical dominance. Note that a potato is a tuber, which is an underground stem modified for starch storage. As with an ordinary shoot, the potato has a terminal bud (containing the shoot apical meristem) or "eye" and several axillary (lateral) buds. After a long period of storage, the terminal bud usually sprouts but the other buds do not. However, if the potato is sliced into sections, one bud to a section, the axillary buds develop just as quickly as the terminal bud (Figure \(8\)).
As will be discussed in the Abscisic Acid section, abscisic acid in the lateral buds inhibits production of auxin, and removal of the apical bud will release this inhibition of auxin, allowing the lateral buds to begin growing.
Flowering and Fruit Development
Auxins promote flowering and fruit setting and ripening. Pollination of the flowers of angiosperms initiates the formation of seeds. As the seeds mature, they release auxin to the surrounding flower parts, which develop into the fruit that covers the seeds.
Prevention of Abscission
Some plants drop leaves and fruits in response to changing seasons (based on temperatures, photoperiod, water, or other environmental conditions). This process is called abscission, and is regulated by interactions between auxin and ethylene. During the growing season, the young leaves and fruits produce high levels of auxin, which blocks activity of ethylene; they thus remain attached to the stem. As the seasons change, auxin levels decline and permit ethylene to initiate senescence, or aging (see Ethylene).
Figure \(9\) demonstrates the role of auxin in abscission. If the blade of the leaf is removed, as shown in the figure, the petiole remains attached to the stem for a few more days. The removal of the blade seems to be the trigger as an undamaged leaf at the same node of the stem remains on the plant much longer, in fact, the normal length of time. If, however, auxin is applied to the cut end of the petiole, abscission of the petiole is greatly delayed.
Mechanisms of Auxin Action
Auxin effects are mediated by two different pathways: immediate, direct effects on the cell and turning on of new patterns of gene expression. The arrival of auxin in the cytosol initiates such immediate responses as changes in the concentration of and movement of ions in and out of the cell and reduction in the redistribution of PIN proteins. At the cellular level, auxin generally increases the rate of cell division and longitudinal cell expansion. Some of the direct effects of auxin may be mediated by its binding to a cell-surface receptor designated ABP1 ("Auxin-binding protein 1").
Many auxin effects are mediated by changes in the transcription of genes. Auxin enters the nucleus and binds to its receptor, a protein called TIR1 ("transport inhibitor response protein 1") which now can bind to proteins responsible for attaching ubiquitin to one or another of several Aux/IAA proteins. This triggers the destruction of the Aux/IAA proteins by proteasomes. Aux/IAA proteins normally bind transcription factors called auxin response factors (ARF) preventing them from activating the promoters and other control sequences of genes that are turned on (or off) by auxin. Destruction of the Aux/IAA proteins relieves this inhibition, and gene transcription begins.
This mechanism is another of the many cases in biology where a pathway is turned on by inhibiting the inhibitor of that pathway (a double-negative is a positive). The presence in the cell of many different Aux/IAA proteins (29 in Arabidopsis), many different ARFs (23 in Arabidopsis) and several (~4) TIR1-like proteins provides a logical basis for mediating the different auxin effects that are described here, but how this is done remains to be discovered.
Commercial Applications of Auxins
Commercial use of auxins is widespread in for propagation in nurseries, crop production, and killing weeds. Horticulturists may propagate desirable plants by cutting pieces of stem and placing them base down in moist soil. Eventually adventitious roots grow out at the base of the cutting. The process can often be hastened by treating the cuttings with a solution or powder containing a synthetic auxin.
Applying synthetic auxins to tomato plants in greenhouses promotes normal fruit development. Fruit growers often apply auxin sprays to cut down the loss of fruit from premature dropping. Additionally, outdoor application of auxin promotes synchronization of fruit setting and dropping to coordinate the harvesting season. Fruits such as seedless cucumbers can be induced to set fruit by treating unfertilized plant flowers with auxins.
Synthetic auxins are widely used as herbicides. Examples include 2,4-dichlorophenoxy acetic acid (2,4-D) and 2,4,5-trichlorophenoxy acetic acid (2,4,5-T), shown in Figure \(10\). 2,4-D and its many variants are popular because they are selective herbicides, killing broad-leaved eudicots but not narrow-leaved monocots. (No one knows the basis of this selectivity). Why should a synthetic auxin kill the plant? It turns out that the auxin influx transporter works fine for 2,4-D, but that 2,4-D cannot leave the cell through the efflux transporters. Perhaps it is the resulting accumulation of 2,4-D within the cell that kills it. A mixture of 2,4,-D and 2,4,5-T was the "agent orange" used by the U.S. military to defoliate the forest in parts of South Vietnam. Because of health concerns, 2,4,5-T is no longer used in the U.S.
Attributions
Curated and authored by Melissa Ha from the following sources: | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/39%3A_Sensory_Systems_in_Plants/39.05%3A_Hormones_and_Sensory_Systems/39.5.01%3A_Auxin.txt |
Learning Objective
Identify the locations of synthesis, transport, and actions of cytokinins.
Cytokinins are plant hormones that promote cytokinesis (cell division) are derivatives of the purine adenine. (They are not to be confused with cytokines.) The effect of cytokinins was first reported when it was found that adding the liquid endosperm of coconuts to developing plant embryos in culture stimulated their growth. Without the cytokinins from the endosperm, plant cells would not divide by mitosis. Almost 200 naturally occurring or synthetic cytokinins are known to date. Zeatin is an example of naturally occurring cytokinin (Figure \(1\)), and kinetin is an example of a synthetic cytokinin.
Cytokinins are synthesized in roots tips and other young structures where cell division is occurring such as embryos and fruits. They are also produced by wounded tissue. Cytokinins are transported through the xylem.
Actions of Cytokinins
One of the clearest examples of cytokinin stimulating cell division involves seed germination. The endosperm of monocot seeds, such as corn (maize), contains large stores of the precursor to the cytokinin zeatin. When the corn kernel germinates, zeatin moves from the endosperm to the root tip where it stimulates vigorous mitosis (Figure \(2\)).
Plant development is controlled by multiple hormones working together or balancing each other's effects. Cytokinins play an important role in plant development. They are involved in leaf formation, and they delay senescence in leaf tissues. Cytokinins also play a role in chloroplast development.
Cytokinins often counter the effects of auxin when regulating shoot and root development. Cytokinins inhibit apical dominance by stimulating axillary bud development, having the opposite effect as auxin. They inhibit the formation of lateral roots while auxin initiates lateral roots. When cytokinins are applied to a callus (mass of undifferentiated cells), shoots form. If auxin is applied, roots form. If the two hormones are applied in equal amounts, much the rate of cell division increases, but the callus does not produce distinct shoots and roots.
With respect to mediating roots gravitropism, however, the effect of cytokinin is similar to that of auxin. When a root is turned on its side, cytokinins accumulate on the lower side, inhibiting elongation there. As the upper surface of the root elongates, it bends downwards.
Mechanism of Cytokinin Action
Like auxins, cytokinin can cause changes in gene expression. To begin this process, a cytokinin binds to a receptor protein embedded in the plasma membrane of the cell. The internal portion of the receptor then attaches a phosphate group to a protein in the cytosol. This protein moves into the nucleus where it activates one or more nuclear transcription factors, which then bind to the promoters of genes. Transcription of these genes produces mRNAs that move out into the cytosol. Translation of these mRNAs produces the proteins that enable the cell to carry out its cytokine-induced function.
Attributions
Curated and authored by Melissa Ha from the following sources:
39.5.03: Gibberellins
Learning Objectives
• Identify the locations of synthesis, transport, and actions of gibberellins.
• Interpret and predict the outcome of an experiment demonstrating the action of gibberellins.
• Describe the commercial applications of gibberellins.
During the 1930s Japanese scientists isolated a growth-promoting substance from cultures of a fungus that parasitizes rice plants. They called it gibberellin. Gibberellins (GAs) are a group of about 125 closely related plant hormones synthesized in the root and stem apical meristems, young leaves, and seed embryos. They are likely transported through the vascular tissue. One of the most active gibberellins - and one found as a natural hormone in the plants themselves - is gibberellic acid (GA; figure \(1\)).
Actions of Gibberellins
Several aspects of plant growth involve GAs, including stimulating shoot elongation, seed germination, and fruit and flower maturation. Other effects of GAs include gender expression (also see Ethylene) and the delay of senescence in leaves and fruit. Synthesis of gibberellins also helps grapevines climb up toward the light by causing meristems that would have developed into flowers to develop into tendrils instead.
Shoot elongation in this case results from both cell division and cell elongation. When applied in low concentrations to a bush or "dwarf" bean, the stem begins to grow rapidly. The length of the internodes becomes so great that the plant becomes indistinguishable from climbing or "pole" beans. GA seems to overcome the genetic limitations in many dwarf varieties.
Gibberellins break dormancy (a state of inhibited growth and development) in the seeds of plants that require exposure to cold or light to germinate. The seeds of some plant species rely on the imbibition (intake) of water to initiate germination. Intake of water activates gibberellins, which then signals to transcribe the gene encoding amylase, an enzyme that breaks down starches stored in the seed into simple sugars (note these final steps are identical to what occurs in phytochrome-regulated germination). Gibberellins also stimulate cell elongation of young roots during germination. When water is absent, germination in this pathway is blocked by a hormone called abscisic acid, which inhibits the activity of gibberellins. Thus gibberellins and abscisic acid act in opposition in regulating the the germination response.
Many plant species first produce a basal rosette of leaves. When daylength increase or the weather becomes cold, they bolt, producing a long stalk. Eventually, flowers and then fruits develop on this stalk. Gibberellins are responsible for inducing bolting (figure \(2\).
Mechanism of Gibberellin Action
Like auxins, gibberellins generally increase the rate of cell division and longitudinal cell expansion. Gibberellins also exert their effects by altering gene transcription through a mechanism similar to auxin in that a pathway is turned on by inhibiting the inhibitor of that pathway (a double-negative is a positive). First, Gibberellin enters the cell and binds to a soluble receptor protein called GID1 ("gibberellin-insensitive dwarf mutant 1") which now can bind to a complex of proteins (SCF) responsible for attaching ubiquitin to one or another of several DELLA proteins. This triggers the destruction of the DELLA proteins by proteasomes. DELLA proteins normally bind gibberellin-dependent transcription factors, a prominent one is designated PIF3/4, preventing them from binding to the DNA of control sequences of genes that are turned on by gibberellin (also see Shade Avoidance and Etiolation).
The dwarf varieties of rice and wheat carry mutations related to GAs. In the case of rice, the mutation interfere with the synthesis of their gibberellins. The wheat mutation reduces is in the gene coding for a DELLA protein and reduce the plant's ability to respond to its own gibberellins. Dwarf varieties of sorghum and more recently maize (corn) also exist, but in these cases, the mutation interferes with auxin transport, not gibberellin activity.
Commercial Applications of Gibberellins
Gibberellin application assists with seedless grape production. Seedless grapes are obtained through standard breeding methods and contain inconspicuous seeds that fail to develop. Because GAs are produced by the seeds, and because fruit development and stem elongation are under GA control, these varieties of grapes would normally produce small fruit in compact clusters. Maturing grapes are routinely treated with GA to promote larger fruit size, as well as looser bunches (longer stems), which reduces the instance of mildew infection (Figure \(3\)). Like auxins, GAs can be used commercially to induce fruit development in a variety of species.
Gibberellins have a few other commercial applications. They can be applied to artificially induce bolting and flowering, such that plants produce seeds earlier. Addition of gibberellic acid to the winter buds of peach trees helps break dormancy. In urban areas, GA antagonists are sometimes applied to trees under power lines to control growth and reduce the frequency of pruning.
Attributions
Curated and authored by Melissa Ha from the following sources: | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/39%3A_Sensory_Systems_in_Plants/39.05%3A_Hormones_and_Sensory_Systems/39.5.02%3A_Cytokinins.txt |
Learning Objectives
• Relate the chemical structure of ethylene to its mode of transport.
• Identify the locations of synthesis and actions of ethylene.
• Describe the commercial applications of ethylene.
Ethylene differs from other plant hormones in that it is a smaller and simpler molecule that is a volatile gas (Figure \(1\)). Hundreds of years ago, when gas street lamps were installed in city streets, trees that grew close to lamp posts developed twisted, thickened trunks and shed their leaves earlier than expected. These effects were caused by ethylene volatilizing from the lamps. Aging tissues, such as those that are wilting or ripening, and nodes of stems produce ethylene.
Actions of Ethylene
Ethylene has many functions, and its major functions are associated with senescence, or aging. This includes fruit ripening, flower wilting, and leaf and fruit abscission. Ethylene also promotes germination in some cereals and sprouting of bulbs and potatoes. It is responsible for drooping of leaves and sprouting of potato buds. In monoecious plants, ethylene promotes the production of female flowers where as gibberellic acid promotes male flower production. Ethylene mediates the triple response, which makes the shoots of seedlings that are buried under debris grow short and wide as well as bend horizontally. This makes it possible for the shoot to push through the debris. Ethylene causes stem elongation in rice and other plants that are submerged in water. It promotes the breakdown of abscisic acid (ABA) and thus relieves ABA's inhibition of gibberellic acid.
Fruit Ripening
As they approach maturity, many fruits (e.g., apples, oranges, avocados) release ethylene. During fruit ripening, ethylene stimulates the conversion of starch and acids to sugars. Some people store unripe fruit, such as avocados, in a sealed paper bag to accelerate ripening; the gas released by the first fruit to mature will speed up the maturation of the remaining fruit.
Abscission
Ethylene induces the abscission of leaves, fruits, and flower petals. When auxin levels decline, ethylene triggers senescence and ultimately programmed cell death at the site of leaf attachment to the stem. A special layer of cells — the abscission layer (abscission zone) — forms at the base of the petiole or fruit stalk (Figure \(2\)). In petioles of some plants, there are two parts of the abscission layer: the more distal separation layer and more proximal protective layer. Before abscission occurs, nutrients are absorbed into the stem so that they are not lost with the leaf. As the separation layer breaks down, the leaf breaks free at this point and leaf falls to the ground in a controlled manner without harming the rest of the plant. The protective layer, which was reinforced with suberin, serves as a seal.
Leaf abscission is particularly important for temperate deciduous trees in the autumn. This is a vital response to the onset of winter when ground water is frozen - and thus cannot support transpiration - and snow load would threaten to break any branches still in leaf.
In drought conditions, the immediate response is closing stomata (see Abscisic Acid). However, because closed stomata prevent gas exchange, plants will die if the stomata remain closed for too long. Thus if a drought persists for too long, the plant will begin sacrificing certain areas by allowing the leaves or stems to die in localized regions. This process may be regulated by ethylene, which can induce localized cell death under certain conditions.
Mechanism of Ethylene Action
At a cellular level, ethylene can inhibit or promote cell division. It sometimes inhibits cell expansion. In other circumstances, it stimulates lateral cell expansion. The presence of ethylene is detected by transmembrane receptors in the endoplasmic reticulum (ER) of cells. Binding of ethylene to these receptors unleashes a signaling cascade that leads to activation of transcription factors and the turning on of gene transcription.
Commercial Applications of Ethylene
Ethylene is widely used in agriculture. Commercial fruit growers can buy equipment to generate ethylene so that their harvest ripens quickly and uniformly. Horticulturalists inhibit leaf dropping in ornamental plants by removing ethylene from greenhouses using fans and ventilation.
Attributions
Curated and authored by Melissa Ha from the following sources: | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/39%3A_Sensory_Systems_in_Plants/39.05%3A_Hormones_and_Sensory_Systems/39.5.04%3A_Ethylene.txt |
Learning Objectives
• Identify the locations of synthesis, transport, and actions of abscisic acid.
• Describe how ABA interacts with other plant hormones.
The plant hormone abscisic acid (ABA) was was once thought to be responsible for abscission; however, this is now known to be incorrect. Instead, ABA accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures, or shortened day lengths. Unlike animals, plants cannot flee from potentially harmful conditions like drought, freezing, exposure to salt water or salinated soil, and ABA plays in mediating adaptations of the plant to stress. Abscisic acid (Figure \(1\)) resembles the carotenoid zeaxanthin (Figure \(2\)), from which it is ultimately synthesized. It is produced in mature leaves and roots and transported through the vascular tissue.
Maintaining Dormancy
Seed Maturation and Inhibition of Germination
Seeds are not only important agents of reproduction and dispersal, but they are also essential to the survival of annual and biennial plants. These angiosperms die after flowering and seed formation is complete. Abscisic acid is essential for seed maturation and also enforces a period of seed dormancy, by blocking germination and promoting the synthesis of storage proteins. It is important the seeds not germinate prematurely during unseasonably mild conditions prior to the onset of winter or a dry season. As the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring. As discussed in the Environmental Responses chapter, other environmental cues such as exposure to a cold period, light, or water are often also needed to for germination to occur.
Interestingly, mangrove species with viviparous germination, meaning that seeds germinate while still attached to the parent plant have reduced levels of ABA during embryo formation, providing further evidence of ABA's role in maintain seed dormancy (Farnsworth and Farrant 1998, Am J. Bot.). These mangroves are adapted to drop germinated seeds into surrounding water to be dispersed (Figure \(3\)).
Bud Dormancy
Another effect of ABA is to promote the development of winter buds; it mediates the conversion of the apical meristem into a dormant bud. The newly developing leaves growing above the meristem become converted into stiff bud scales that wrap the meristem closely and will protect it from mechanical damage and drying out during the winter. Abscisic acid in the bud also acts to enforce dormancy so if an unseasonably warm spell occurs before winter is over, the buds will not sprout prematurely. Only after a prolonged period of cold or the lengthening days of spring (photoperiodism) will bud dormancy be lifted.
Response to Water Stress
Stomatal Closure
Abscisic acid also regulates the short-term drought response. Recall that stomata are pores in the leaf and are surrounded by a pair of guard cells. Much of the water taken up by a plant is lost as water vapor exists stomata. Low soil moisture causes an increase in ABA, which causes stomata to close, reducing water loss. Note that stomatal closure also prevents exchange of oxygen and carbon dioxide, which is necessary for efficient photosynthesis (see Photorespiration and Phytosynthetic Pathways). The response to abscisic acid occurs even if blue light is present; that is, signaling from drought via ABA overrides the signaling from blue light to open stomata. See Transport for more details about stomatal opening and closure.
Cellular Protection from Dehydration
Abscisic acid turns on the expression of genes encoding proteins that protect cells - in seeds as well as in vegetative tissues - from damage when they become dehydrated.
Interactions with Other Hormones
At a cellular level, abscisic acid inhibits both cell division and cell expansion. It often opposes the growth-inducing effects of auxin and gibberellic acid. For example, abscisic acid prevents stem elongation probably by its inhibitory effect on gibberellic acid. In maintaining apical dominance, however, ABA synergizes with auxin. Abscisic acid moves up from the roots to the stem (opposite the flow of auxin) and suppresses the development of axillary buds. The result is inhibition of branching (maintaining apical dominance).
Attributions
Curated and authored by Melissa Ha from the following sources: | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/39%3A_Sensory_Systems_in_Plants/39.05%3A_Hormones_and_Sensory_Systems/39.5.05%3A_Abscisic_Acid.txt |
Learning Objectives
• Identify several signaling molecules beyond the five major plant hormones and describe their effects.
• Distinguish between the hypersensitive response and systemic acquired response.
• Explain the mechanisms by which signaling compounds aid in plant defense against pathogens and herbivores.
Recent research has discovered a number of compounds that also influence plant development. Their roles are less understood than the effects of the major hormones described so far.
Brassinosteroids
Brassinosteroids (Figure \(1\)) are synthesized primarily in young tissues are important to many developmental and physiological processes. In fact, many sources considere them the sixth major plant hormones. Unlike the hormones discussed previously, brassinosteroids do not travel far from their site of synthesis. Signals between these compounds and other hormones, notably auxin and GAs, amplifies their physiological effect. Apical dominance, seed germination, gravitropism, lateral root formation, differentiation of cells in the vascular tissue, and resistance to freezing are all positively influenced by brassinosteroids. Root growth and fruit dropping are inhibited by steroids.
Systemin
Systemin, named for the fact that it is distributed systemically (everywhere) in the plant body upon production, is a short polypeptide that activates plant responses to wounds from herbivores (animals that feed on plant parts). It causes the plant to produce jasmonic acid (see below).
Jasmonates
Jasmonates play a major role in defense responses to herbivory (Figure \(2\)). Their levels increase when a plant is wounded by an herbivore, resulting in an increase in toxic secondary metabolites. For example, jasmonic acid (Figure \(3\)) also induces transcription of protease inhibitors. Protease inhibitors both taste bad and prevent breakdown of proteins in the herbivore’s gut, thus making the insect sick and deterring further herbivory. Jasmonates also contribute to the production of volatile compounds that attract natural enemies of herbivores. Chewing of tomato plants by caterpillars leads to an increase in jasmonic acid levels, which in turn triggers the release of volatile compounds that attract predators of the pest. Jasmonates also elicit the synthesis of volatile compounds that attract parasitoids, which are insects that spend their developing stages in or on another insect, and eventually kill their host.
Jasmonates also work with systemin to mediate responses to drought, damage by ground-level ozone, and ultraviolet light.
Salicylic Acid
Salicylic acid resembles aspirin (Figure \(4\)) and is important for plant defense. It initiates the a systemic (whole body) response called the systemic acquired response (SAR) as a response to infection by parasites or pathogens. When a parasite or pathogen infects a cell, there is an specific, localized response called the hypersensitive response (HR). Following this very localized response, the plant initiates a systemic (whole body) response called the systemic acquired response (SAR). Salicylic acid is produced and converted to methyl salicylate (Figure \(4\)) inducing the SAR in response to the HR. The SAR activates transcription of general “pathogenesis-resistance” genes, which are not pathogen-specific (unlike in the hypersensitive response), but serve as general defense against pathogenic infection. The SAR is slower than the hypersensitive response, and also differs in that it is systemic instead of localized to the site of the infection.
Similar to jasmonic acid, salicylic acid can mediates defense against insect herbivores. It is directly toxic to some herbivores. Additionally, in response to herbivory, salicylic acid can be converted to methyl salicylate, which is released as a gas. This volatile compound can attract natural predators and parasites of the herbivores.
Some plants, such as skunk cabbage (Figure \(5\)) and elephant yam, are adapted to flower while snow still covers the ground. Salicylic acid mediates their ability to produce heat to melt the snow around them. Such plants are thus called thermogenic ("heat producing").
Oligosaccharins
Oligosaccharins are short chains of simple sugars that play a role in plant defense against bacterial and fungal infections. They act locally at the site of injury, and can also be transported to other tissues.
Strigolactones
Strigolactones (Figure \(6\)) promote seed germination in some species and inhibit lateral apical development in the absence of auxins. Strigolactones also play a role in the establishment of mycorrhizae, a mutualistic association of plant roots and fungi.
Florigen
Florigen is a systemic signal that initiates flowering. It is also involved in the formation of storage organs and contributes to plant architecture. It is synthesized in leaves and transported to the shoot apical meristem (SAM) where it promotes flowering in response to daylength cues. At the molecular level, florigen is represented as a protein product encoded by the FLOWERING LOCUS T (FT) gene, which is highly conserved (occurs/has a similar genetic sequence in) across flowering plants.
Florigen is considered one of the important targets for crop improvement. Regulation of flowering time is an important target for plant breeding because the control of flowering to a favorable time provides successful grain production in a given cropping area. Flowering at unfavorable seasons causes loss of yield due to insufficient growth of photosynthetic organs or poor fertility due to heat or cold stress during reproduction. Thus, understanding florigen function can contribute to novel breeding techniques in crops to produce cultivars that can start their reproductive stage at optimal seasons.
Supplemental Reading
Filgueiras, C. C., Martins, A. D., Pereira, R. V., & Willett, D. S. (2019). The Ecology of Salicylic Acid Signaling: Primary, Secondary and Tertiary Effects with Applications in Agriculture. International journal of molecular sciences, 20 (23), 5851. https://doi.org/10.3390/ijms20235851
Attributions
Curated and authored by Melissa Ha from the following sources: | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/39%3A_Sensory_Systems_in_Plants/39.05%3A_Hormones_and_Sensory_Systems/39.5.06%3A_Other_Signaling_Molecules.txt |
Growth in plants occurs chiefly at meristems where rapid mitosis provides new cells. As these cells differentiate, they provide new plant tissue.
Stem Growth
In stems, mitosis in the apical meristem of the shoot apex (also called the terminal bud) produces cells that enable the stem to grow longer and, periodically, cells that will give rise to leaves. The point on the stem where leaves develop is called a node. The region between a pair of adjacent nodes is called the internode. The internodes in the terminal bud are very short so that the developing leaves grow above the apical meristem that produced them and thus protect it. New meristems, the lateral buds, develop at the nodes, each just above the point where a leaf is attached. When the lateral buds develop, they produces new stem tissue, and thus branches are formed.
Under special circumstances (such as changes in photoperiod), the apical meristem is converted into a flower bud. This develops into a flower. The conversion of the apical meristem to a flower bud "uses up" the meristem so that no further growth of the stem can occur at that point. However, lateral buds behind the flower can develop into branches.
The drawing is of a typical woody dicot, the horse chestnut, as seen during the dormant season. The leaves have dropped off, leaving a leaf scar; the dots inside each leaf scar show where the vascular bundles (xylem and phloem) had entered the petiole of the leaf. A flower had been produced the season before, so that during the season just ended two branches had grown out on either side of the flower bud scar. Lenticels are openings that allow oxygen and carbon dioxide to diffuse between the living cells of the stem and the air.
The growth of roots in described in a separate page,
40.2D: Flowering
The flowering plants (angiosperms) go through a phase of vegetative growth producing more stems and leaves and a flowering phase where they produce the organs for sexual reproduction. In "annuals", like the snapdragon, the vegetative phase begins with germination of the seed. Flowering follows and ends with the senescence and death of the plant. In biennials, the vegetative phase takes up the first year; flowering followed by death occurs the second year. In perennials, flowering typically occurs year after year when conditions are appropriate.
Vegetative growth of the above-ground part of the plant — the shoot — occurs at the apical meristem. This is a mass of undifferentiated cells at the tip of the stem. Mitosis of these cells produces cells that differentiate to form more stem, leaves and secondary meristems. Also called lateral buds, these form in the axils of the leaves and will form branches.
The Signal to Flower
Flowering involves the conversion of the apical meristem into a floral meristem, from which all the parts of the flower will be produced. Signals that change the fate of the apical meristem include:
• maturity of the plant
• temperature
• the arrival of the plant hormone gibberellin
• for many plants, photoperiod - the relative length of day and night.
Temperature
Many annual plants (e.g., winter wheat) and biennial plants have their time of flowering delayed unless they have undergone a preceding period of wintertime cold. The change brought about by this prolonged exposure to the cold is called vernalization.
In the "model" plant Arabidopsis thaliana, vernalization works like this.
• A gene designated Flowering Locus C (FLC) encodes a transcription factor that blocks the expression of the genes needed for flowering.
• The level of FLC mRNA is high in the fall.
• But with the onset of cold temperatures, production of an antisense transcript of FLC (called COOLAIR) increasesas does, later, a sense transcript of part of the FLC gene.
• Both of these RNAs are non-coding; that is, they are not translated into protein.
• But they cooperate in suppressing the production of FLC mRNA and its translation into FLC protein.
• With the arrival of spring, there is no FLC protein remaining to suppress flowering so flowering can begin.
The buds of many species of woody angiosperms found in temperate climates, such as apples and lilacs, also need a preceding period of cold weather before they can develop into flowers. So these plants cannot be grown successfully at lower latitudes because the winters never get cold enough (a few days at 0–10°C). This bud dormancy is localized. Prior chilling of one bud on a lilac stem enables it to flower while the other, nonchilled, buds on the stem remain dormant.
Photoperiod
Photoperiod is detected in the leaves. The cocklebur, drawn here, needs at least 8.5 hours of darkness in order to flower. Even if only a part of one leaf is exposed to the correct photoperiod, the entire plant will bloom (middle figure).
The leaves produce a chemical signal called florigen that is transmitted to the apical meristems to start their conversion into floral meristems. The right-hand drawing shows that grafting a cocklebur (B) that receives the required period of darkness to one (A) that does not causes flowering in both. Evidently the florigen signal passes from B to A through their connected vascular systems.
The chemical nature of florigen has been sought for decades. The most recent evidence suggests that at least one component is the protein encoded by the gene FLOWERING LOCUS T (FT).
Converting the Apical Meristem to a Floral Meristem
In the nucleus of the meristem cells, the FT protein binds to the transcription factor FD and turns on the expression of genes needed for flowering, e.g., APETALA1 and LEAFY.
Structure of the Flower
The floral meristem differentiates into four concentric groups of cells that form the four parts of the flower.
1. The cells in whorl 1 develop into a whorl of sepals. These form at the lowest level. Collectively they make up the calyx.
2. Whorl 2 forms above the calyx, forming the petals. Collectively these make up the corolla of the flower (the part that most ornamentals are grown for).
3. Whorl 3 develops into the stamens, the male reproductive organs.
4. The innermost whorl, 4, forms carpels, the female reproductive organs. Carpels often fuse to form a single structure, which some botanists call the pistil.
What triggers the various parts of the floral meristem to enter one or another of these four developmental pathways?
The ABC Model of Flower Development
Genetic analysis of mutants especially those found in the dicots Arabidopsis thaliana and in the snapdragon (Antirrhinum) support the ABC model of flowering. This model postulates a group of genes that encode the transcription factors needed to turn on the genes for sepal, petal, etc. development. The "master switches" fall into 3 groups: A, B, and C.
These are the rules:
• Cells in which only A genes are expressed develop into sepals. This occurs at the lowest level of the floral meristem.
• Cells in which both A and B genes are expressed develop into petals. This occurs at the next higher level.
• Expression of B and C genes turns on the developmental program to form stamens.
• Expression of C genes alone turns on the development of carpels in the innermost band of cells.
Examples of A, B and C group genes involved in flowering - these have been identified in Arabidopsis thaliana
A group APETALA1 (AP1) and
APETALA2 (AP2)
B group APETALA3 (AP3) and
PISTILLATA (PI)
C group AGAMOUS (AG)
The transcription factor LEAFY plays a major role in turning on the A, B, and C group genes in the appropriate locations.
• The LEAFY protein alone turns on AP1 in whorls 1 and 2.
• LEAFY plus a protein encoded by the gene UFO (for "unusual floral organs") turn on AP3 in whorls 2 and 3.
• LEAFY and a second, still unidentified, protein turn on AG in whorls 3 and 4.
If LEAFY protein alone is sufficient to turn on AP1, why isn't AP1 expressed in all four whorls?
The answer: AGAMOUS blocks the expression of AP1, so any cell expressing AGAMOUS cannot express AP1. In fact, the antagonism is reciprocal: AP2 in whorls 1 and 2 (A group) inhibits AGAMOUS so the gene expression in whorls 3 and 4 remains distinct from that in whorls 1 and 2.
The proteins encoded by APETALA3 and PISTILLATA (Group B) form a heterodimer that binds to
• the APETALA1 protein to form petals
• the AGAMOUS protein to form stamens
Aided by a fourth transcription factor encoded by the gene SEPALLATA3, these quaternary complexes bind to specific sequences of DNA turning on the expression of the various genes needed to form whorls 2 and 3. Further research may reveal similar behavior for the other genes.
SEPALLATA3 (SEP3) is one of four SEP genes in Arabidopsis. If all but SEP4 are inactivated, a flower with only sepals is formed (hence the name). If all four are inactivated, no flowers are formed at all.
So formation of a flower requires a cascade of sequential gene activity that gradually converts a mass of undifferentiated cells (the apical meristem) into the parts of a flower. The genes encode transcription factors that act as master switches, turning on (or off) downstream genes that ultimately make each part of the flower in its appropriate location. This same strategy of genetic control of developmental pathways is seen in animal development. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/40%3A_Plant_Reproduction/40.01%3A_Reproductive_Development/40.1A%3A_Plant_Growth.txt |
Learning Objectives
• Outline the components of a flower and their function
The lifecycle of angiosperms follows the alternation of generations. In the angiosperm, the haploid gametophyte alternates with the diploid sporophyte during the sexual reproduction process of angiosperms. Flowers contain the plant’s reproductive structures.
Flower Structure
A typical flower has four main parts, or whorls: the calyx, corolla, androecium, and gynoecium. The outermost whorl of the flower has green, leafy structures known as sepals, which are collectively called the calyx, and help to protect the unopened bud. The second whorl is comprised of petals, usually brightly colored, collectively called the corolla. The number of sepals and petals varies depending on whether the plant is a monocot or dicot. Together, the calyx and corolla are known as the perianth. The third whorl contains the male reproductive structures and is known as the androecium. The androecium has stamens with anthers that contain the microsporangia. The innermost group of structures in the flower is the gynoecium, or the female reproductive component(s). The carpel is the individual unit of the gynoecium and has a stigma, style, and ovary. A flower may have one or multiple carpels.
If all four whorls are present, the flower is described as complete. If any of the four parts is missing, the flower is known as incomplete. Flowers that contain both an androecium and a gynoecium are called perfect, androgynous, or hermaphrodites. There are two types of incomplete flowers: staminate flowers contain only an androecium; and carpellate flowers have only a gynoecium.
If both male and female flowers are borne on the same plant (e.g., corn or peas), the species is called monoecious (meaning “one home”). Species with male and female flowers borne on separate plants (e.g., C. papaya or Cannabis)are termed dioecious, or “two homes.” The ovary, which may contain one or multiple ovules, may be placed above other flower parts (referred to as superior); or it may be placed below the other flower parts (referred to as inferior).
Male Gametophyte
The male gametophyte develops and reaches maturity in an immature anther. In a plant’s male reproductive organs, development of pollen takes place in a structure known as the microsporangium. The microsporangia, usually bi-lobed, are pollen sacs in which the microspores develop into pollen grains.
Within the microsporangium, the microspore mother cell divides by meiosis to give rise to four microspores, each of which will ultimately form a pollen grain. An inner layer of cells, known as the tapetum, provides nutrition to the developing microspores, contributing key components to the pollen wall. Mature pollen grains contain two cells: a generative cell and a pollen tube cell. The generative cell is contained within the larger pollen tube cell. Upon germination, the tube cell forms the pollen tube through which the generative cell migrates to enter the ovary. During its transit inside the pollen tube, the generative cell divides to form two male gametes. Upon maturity, the microsporangia burst, releasing the pollen grains from the anther.
Each pollen grain has two coverings: the exine (thicker, outer layer) and the intine. The exine contains sporopollenin, a complex waterproofing substance supplied by the tapetal cells. Sporopollenin allows the pollen to survive under unfavorable conditions and to be carried by wind, water, or biological agents without undergoing damage.
Female Gametophyte (Embryo Sac)
The overall development of the female gametophyte has two distinct phases. First, in the process of megasporogenesis, a single cell in the diploid megasporangium undergoes meiosis to produce four megaspores, only one of which survives. During the second phase, megagametogenesis, the surviving haploid megaspore undergoes mitosis to produce an eight-nucleate, seven-cell female gametophyte, also known as the megagametophyte, or embryo sac. The polar nuclei move to the equator and fuse, forming a single, diploid central cell. This central cell later fuses with a sperm to form the triploid endosperm. Three nuclei position themselves on the end of the embryo sac opposite the micropyle and develop into the antipodal cells, which later degenerate. The nucleus closest to the micropyle becomes the female gamete, or egg cell, and the two adjacent nuclei develop into synergid cells. The synergids help guide the pollen tube for successful fertilization, after which they disintegrate. Once fertilization is complete, the resulting diploid zygote develops into the embryo; the fertilized ovule forms the other tissues of the seed.
A double-layered integument protects the megasporangium and, later, the embryo sac. The integument will develop into the seed coat after fertilization, protecting the entire seed. The ovule wall will become part of the fruit. The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization.
Key Points
• A typical flower has four main parts, or whorls: the calyx ( sepals ), corolla (petals), androecium (male reproductive structure), and gynoecium (female reproductive structure).
• Angiosperms that contain both male and female gametophytes within the same flower are called complete and are considered to be androgynous or hermaphroditic.
• Angiosperms that contain only male or only female gametophytes are considered to be incomplete and are either staminate (contain only male structures) or carpellate (contain only female structures) flowers.
• Microspores develop in the microsporangium and form mature pollen grains (male gametophytes), which are then used to fertilize female gametophytes.
• During megasporogenesis, four megaspores are produced with one surviving; during megagametogenesism, the surviving megaspore undergoes mitosis to form an embryo sac (female gametophyte).
• The sperm, guided by the synergid cells, migrates to the ovary to complete fertilization; the diploid zygote develops into the embryo, while the fertilized ovule forms the other tissues of the seed.
Key Terms
• perianth: the calyx (sepals) and the corolla (petals)
• androecium: the set of a flower’s stamens (male reproductive organs)
• gynoecium: the set of a flower’s pistils (female reproductive organs) | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/40%3A_Plant_Reproduction/40.03%3A_Structure_and_Evolution_of_Flowers/40.3C%3A_Sexual_Reproduction_in_Angiosperms.txt |
Learning Objectives
• Determine the differences between self-pollination and cross-pollination, and describe how plants have developed ways to avoid self-pollination
Pollination: An Introduction
In angiosperms, pollination is defined as the placement or transfer of pollen from the anther to the stigma of the same or a different flower. In gymnosperms, pollination involves pollen transfer from the male cone to the female cone. Upon transfer, the pollen germinates to form the pollen tube and the sperm that fertilize the egg.
Self-Pollination and Cross-Pollination
Pollination takes two forms: self-pollination and cross-pollination. Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower or another flower on the same plant. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species. Self-pollination occurs in flowers where the stamen and carpel mature at the same time and are positioned so that the pollen can land on the flower’s stigma. This method of pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators. These types of pollination have been studied since the time of Gregor Mendel. Mendel successfully carried out self-pollination and cross-pollination in garden peas while studying how characteristics were passed on from one generation to the next. Today’s crops are a result of plant breeding, which employs artificial selection to produce the present-day cultivars. An example is modern corn, which is a result of thousands of years of breeding that began with its ancestor, teosinte. The teosinte that the ancient Mesoamericans originally began cultivating had tiny seeds, vastly different from today’s relatively giant ears of corn. Interestingly, though these two plants appear to be entirely different, the genetic difference between them is minuscule.
Genetic Diversity
Living species are designed to ensure survival of their progeny; those that fail become extinct. Genetic diversity is, therefore, required so that in changing environmental or stress conditions, some of the progeny can survive. Self-pollination leads to the production of plants with less genetic diversity since genetic material from the same plant is used to form gametes and, eventually, the zygote. In contrast, cross-pollination leads to greater genetic diversity because the male and female gametophytes are derived from different plants. Because cross-pollination allows for more genetic diversity, plants have developed many ways to avoid self-pollination. In some species, the pollen and the ovary mature at different times. These flowers make self-pollination nearly impossible. By the time pollen matures and has been shed, the stigma of this flower is mature and can only be pollinated by pollen from another flower. Some flowers have developed physical features that prevent self-pollination. The primrose employs this technique. Primroses have evolved two flower types with differences in anther and stigma length: the pin-eyed flower and the thrum-eyed flower. In the pin-eyed flower, anthers are positioned at the pollen tube’s halfway point, and in the thrum-eyed flower, the stigma is found at this same location. This allows insects to easily cross-pollinate while seeking nectar at the pollen tube. This phenomenon is also known as heterostyly. Many plants, such as cucumbers, have male and female flowers located on different parts of the plant, thus making self-pollination difficult. In other species, the male and female flowers are borne on different plants, making them dioecious. All of these are barriers to self-pollination; therefore, the plants depend on pollinators to transfer pollen. The majority of pollinators are biotic agents such as insects (bees, flies, and butterflies), bats, birds, and other animals. Other plant species are pollinated by abiotic agents, such as wind and water.
Key Points
• Pollination, the transfer of pollen from flower-to-flower in angiosperms or cone -to-cone in gymnosperms, takes place through self-pollination or cross-pollination.
• Cross-pollination is the most advantageous of the two types of pollination since it provides species with greater genetic diversity.
• Maturation of pollen and ovaries at different times and heterostyly are methods plants have developed to avoid self-pollination.
• The placement of male and female flowers on separate plants or different parts of the plant are also barriers to self-pollination.
Key Terms
• pollination: the transfer of pollen from an anther to a stigma that is carried out by insects, birds, bats, and the wind
• heterostyly: the condition of having unequal male (anther) and female (stigma) reproductive organs
• cross-pollination: fertilization by the transfer of pollen from an anther of one plant to a stigma of another
• self-pollination: pollination of a flower by its own pollen in a flower that has both stamens and a pistil | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/40%3A_Plant_Reproduction/40.04%3A_Pollination_and_Fertilization/40.4A%3A_Pollination_and_Fertilization.txt |
Learning Objectives
• Explain how pollination by insects aids plant reproduction
Bees
Bees are perhaps the most important pollinator of many garden plants and most commercial fruit trees. The most common species of bees are bumblebees and honeybees. Since bees cannot see the color red, bee-pollinated flowers usually have shades of blue, yellow, or other colors. Bees collect energy -rich pollen or nectar for their survival and energy needs. They visit flowers that are open during the day, are brightly colored, have a strong aroma or scent, and have a tubular shape, typically with the presence of a nectar guide. A nectar guide includes regions on the flower petals that are visible only to bees, which help guide bees to the center of the flower, thus making the pollination process more efficient. The pollen sticks to the bees’ fuzzy hair; when the bee visits another flower, some of the pollen is transferred to the second flower. Recently, there have been many reports about the declining population of honeybees. Many flowers will remain unpollinated, failing to bear seeds if honeybees disappear. The impact on commercial fruit growers could be devastating.
Flies
Many flies are attracted to flowers that have a decaying smell or an odor of rotting flesh. These flowers, which produce nectar, usually have dull colors, such as brown or purple. They are found on the corpse flower or voodoo lily (Amorphophallus), dragon arum (Dracunculus), and carrion flower (Stapleia, Rafflesia). The nectar provides energy while the pollen provides protein. Wasps are also important insect pollinators, pollinating many species of figs.
Butterflies and Moths
Butterflies, such as the monarch, pollinate many garden flowers and wildflowers, which are usually found in clusters. These flowers are brightly colored, have a strong fragrance, are open during the day, and have nectar guides. The pollen is picked up and carried on the butterfly’s limbs. Moths, on the other hand, pollinate flowers during the late afternoon and night. The flowers pollinated by moths are pale or white and are flat, enabling the moths to land. One well-studied example of a moth-pollinated plant is the yucca plant, which is pollinated by the yucca moth. The shape of the flower and moth have adapted in a way to allow successful pollination. The moth deposits pollen on the sticky stigma for fertilization to occur later. The female moth also deposits eggs into the ovary. As the eggs develop into larvae, they obtain food from the flower and developing seeds. Thus, both the insect and flower benefit from each other in this symbiotic relationship. The corn earworm moth and Gaura plant have a similar relationship.
Key Points
• Adaptations such as bright colors, strong fragrances, special shapes, and nectar guides are used to attract suitable pollinators.
• Important insect pollinators include bees, flies, wasps, butterflies, and moths.
• Bees and butterflies are attracted to brightly-colored flowers that have a strong scent and are open during the day, whereas moths are attracted to white flowers that are open at night.
• Flies are attracted to dull brown and purple flowers that have an odor of decaying meat.
• Nectar guides, which are only visible to certain insects, facilitate pollination by guiding bees to the pollen at the center of flowers.
• Insects and flowers both benefit from their specialized symbiotic relationships; plants are pollinated while insects obtain valuable sources of food.
Key Terms
• nectar guide: markings or patterns seen in flowers of some angiosperm species that guide pollinators to nectar or pollen | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/40%3A_Plant_Reproduction/40.04%3A_Pollination_and_Fertilization/40.4B%3A_Pollination_by_Insects.txt |
Learning Objectives
• Differentiate among the non-insect methods of pollination
Non-Insect Methods of Pollination
Plants have developed specialized adaptations to take advantage of non-insect forms of pollination. These methods include pollination by bats, birds, wind, and water.
Pollination by Bats
In the tropics and deserts, bats are often the pollinators of nocturnal flowers such as agave, guava, and morning glory. The flowers are usually large and white or pale-colored so that they can be distinguished from their dark surroundings at night. The flowers have a strong, fruity, or musky fragrance and produce large amounts of nectar. They are naturally-large and wide-mouthed to accommodate the head of the bat. As the bats seek the nectar, their faces and heads become covered with pollen, which is then transferred to the next flower.
Pollination by Birds
Many species of small birds, such as hummingbirds and sun birds, are pollinators for plants such as orchids and other wildflowers. Flowers visited by birds are usually sturdy and are oriented in a way to allow the birds to stay near the flower without getting their wings entangled in the nearby flowers. The flower typically has a curved, tubular shape, which allows access for the bird’s beak. Brightly-colored, odorless flowers that are open during the day are pollinated by birds. As a bird seeks energy-rich nectar, pollen is deposited on the bird’s head and neck and is then transferred to the next flower it visits. Botanists determine the range of extinct plants by collecting and identifying pollen from 200-year-old bird specimens from the same site.
Pollination by Wind
Most species of conifers and many angiosperms, such as grasses, maples, and oaks, are pollinated by wind. Pine cones are brown and unscented, while the flowers of wind-pollinated angiosperm species are usually green, small, may have small or no petals, and produce large amounts of pollen. Unlike the typical insect-pollinated flowers, flowers adapted to pollination by wind do not produce nectar or scent. In wind-pollinated species, the microsporangia hang out of the flower, and, as the wind blows, the lightweight pollen is carried with it. The flowers usually emerge early in the spring before the leaves so that the leaves do not block the movement of the wind. The pollen is deposited on the exposed feathery stigma of the flower.
Pollination by Water
Some weeds, such as Australian sea grass and pond weeds, are pollinated by water. The pollen floats on water. When it comes into contact with the flower, it is deposited inside the flower.
Pollination by Deception
Orchids are highly-valued flowers, with many rare varieties. They grow in a range of specific habitats, mainly in the tropics of Asia, South America, and Central America. At least 25,000 species of orchids have been identified.
Flowers often attract pollinators with food rewards, in the form of nectar. However, some species of orchid are an exception to this standard; they have evolved different ways to attract the desired pollinators. They use a method known as food deception, in which bright colors and perfumes are offered, but no food. Anacamptis morio, commonly known as the green-winged orchid, bears bright purple flowers and emits a strong scent. The bumblebee, its main pollinator, is attracted to the flower because of the strong scent, which usually indicates food for a bee. In the process, the bee picks up the pollen to be transported to another flower.
Other orchids use sexual deception. Chiloglottis trapeziformis emits a compound that smells the same as the pheromone emitted by a female wasp to attract male wasps. The male wasp is attracted to the scent, lands on the orchid flower, and, in the process, transfers pollen. Some orchids, like the Australian hammer orchid, use scent as well as visual trickery in yet another sexual deception strategy to attract wasps. The flower of this orchid mimics the appearance of a female wasp and emits a pheromone. The male wasp tries to mate with what appears to be a female wasp, but instead picks up pollen, which it then transfers to the next counterfeit mate.
Key Points
• Flowers that are pollinated by bats bloom at night, tending to be large, wide-mouthed, and pale-colored; they may also give off strong scents.
• Flowers that are pollinated by small birds usually have curved, tubular shapes; birds carry the pollen off on their heads and neck to the next flower they visit.
• Wind-pollinated flowers do not produce scents or nectar; instead, they tend to have small or no petals and to produce large amounts of lightweight pollen.
• Some species of flowers release pollen that can float on water; pollination occurs when the pollen reaches another plant of the same species.
• Some flowers deceive pollinators through food or sexual deception; the pollinators become attracted to the flowers with false promises of food and mating opportunities.
Key Terms
• food deception: a trickery method employed by some species of orchids in which only bright colors and perfume are offered to their pollinators with no food reward | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/40%3A_Plant_Reproduction/40.04%3A_Pollination_and_Fertilization/40.4C%3A_Pollination_by_Bats_Birds_Wind_and_Water.txt |
Learning Objectives
• Explain the steps of embryogensis in eudicots.
• Compare embryogenesis in eudicots versus monocots.
The Seed Plants chapter discussed the fertilization of the egg within the ovule. The zygote ultimately divides to produce the mature embryo, the ovule develops into a seed, and the ovary that contained one or more ovules develops into a fruit. A typical seed contains a seed coat, cotyledons, endosperm, and a single embryo. The development of the embryo occurs through a process called embryogenesis.
Embryogenesis in Eudicots
After fertilization in eudicots, the zygote (Figure \(\PageIndex{1-I}\)) divides to form two cells: the upper apical cell and the lower basal cell . The division of the basal cell gives rise to the suspensor, which attaches the embryo to the micropyle (the pore through which the pollen tube original entered). The suspensor provides a route for nutrition to be transported from the mother plant to the growing embryo. The apical cell also divides, initially producing a proembryo (figure \(\PageIndex{1-II}\)).
As the proembryo continues to divide, it takes a spherical form, called the globular stage (Figures \(\PageIndex{1-III}\) and \(\PageIndex{2a}\)). The globular is the first stage that is considered the embryo proper. Next, cotyledons arise from the embryo proper, forming the heart stage (Figures \(\PageIndex{1-IV}\) and \(\PageIndex{2-4}\)). Cotyledons are embryonic leaf-life structures that function in food storage, food absorption and/or photosynthesis. As the cotyledons elongate, and the base of the embryo thickens, it results a torpedo. (This stage is called the torpedo stage; Figures \(\PageIndex{1-V}\) and \(\PageIndex{2c}\)). Cell division is concentrated at the shoot apical meristem, located at the shoot tip in between the cotyledons, and the root apical meristem at the most basal (bottom) part of the embryo. Most of the suspensor deteriorates during the torpedo stage.
The final stage of embryogenesis results in the mature embryo (Figures \(\PageIndex{1-VI}\) and \(\PageIndex{2d}\)). The mature embryo includes an embryonic root called the radicle. The embryo becomes dormant at this point, halting metabolic activity and cell division. At this point, the seed is ready for dispersal. Growth resumes when the seed germinates and the embryo develops into a seedling.
In some eudicots, the endosperm (triploid tissue that was formed when a sperm fertilized the two polar nuclei) cells divide, and endosperm fills a substantial portion of the mature seed. The endosperm stores nutrients. In other (non-endospermic) eudicots, such as Capsella bursa-pastoris, the endosperm develops initially, but is then digested, and the nutrients are moved into the two cotyledons (Figure \(2\)). After germination, the developing seedling relies on these food reserves stored in the endosperm or cotyledons until the first set of leaves begin photosynthesis.
Embryogenesis in Monocots
The process of embryogenesis in monocots is similar to that of eudicots, but as there is only a single cotyledon, no heart stage occurs. Instead, the embryo proper of the monocot becomes cylindrical at this point in development. The shoot apical meristem, while still present at the shoot tip, is not in between cotyledons in the monocot (because there is only a single cotyledon).
Attribution
Curated and authored by Melissa Ha using 32.2 Pollination and Fertilization from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/40%3A_Plant_Reproduction/40.05%3A_Embryo_Development/40.5.01%3A_Embryogenesis.txt |
Learning Objectives
• Locate the major seed structures and identify the function of each.
• Compare eudicot and monocot seeds.
Eudicot Seeds
The seed is protected by a seed coat that is formed from the integument of the ovule (Figure \(1\)). The seed coat is further divided into an outer coat known as the testa and inner coat known as the tegmen. The hilum is a scar on the outside of the seed where it was attached to the endocarp (inner layer of the fruit wall). The micropyle is a small round structure next to the hilum where the pollen tube entered.
The embryonic axis (root-shoot axis) runs the length of the embryo. On end of the embryonic axis is the plumule, the young shoot apex, which includes the shoot apical meristem and developing leaves (leaf primordia). At the other end of the embryonic axis is the radicle (embryonic root). In some species, the radicle is not apparent in the embryo (in which case the distal end of the root is simply the root tip). The embryonic axis does not include the cotyledons. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl (hypocotyl means “below the cotyledons”). The portion of the embryonic axis between the cotyledon attachment and the shoot tip is the epicotyl (epicotyl means "above the cotyledons; Figures \(\PageIndex{2-3}\))). Some embryos lack a visible epicotyl because the cotyledons are attached to the embryonic axis at the shoot tip.
The two cotyledons in the eudicot seed are connected to the rest of the embryo via vascular tissue (xylem and phloem). In endospermic dicots, the food reserves are stored in the endosperm. During germination, the two cotyledons therefore act as absorptive organs to take up the enzymatically released food reserves. Tobacco (Nicotiana tabaccum), tomato (Solanum lycopersicum), and pepper (Capsicum annuum) are examples of endospermic dicots. In non-endospermic dicots, the triploid endosperm develops normally following double fertilization, but the endosperm food reserves are quickly remobilized and moved into the developing cotyledon for storage. The two halves of a peanut seed (Arachis hypogaea; Figure \(4\)) or a bean (Phaseolus; Figures \(\PageIndex{2-3}\)) and the split peas (Pisum sativum; Figure \(5\)) of split pea soup are individual cotyledons loaded with food reserves.
Because seeds have food reserves to fuel germination, they also are a nutritious food source for people. Studying the nutrient content of seed crops such as beans can be used to increase the nutritive value of the plant using biotechnology. Maria Elena Zavala (Figure \(6\)) is working to combat world hunger by manipulating plants to improve their nutritional qualities. For example, her research with beans is looking at how genetic engineering can be used to make the bean proteins more digestible and nutritious.
Monocot Seeds
The seeds of the most complex monocot family, Poaceae (the grass family), which includes corn and wheat, are highly specialized. The testa and tegmen of the seed coat are fused. The fruit is a caryopsis (grain), a one-seeded fruit in which the fruit wall (pericarp) is fused to the seed coat. Thus, not only are the two layers of the seed coat fused, but the seed coat is fused to the pericarp.
The single cotyledon is called a scutellum; the scutellum also has vascular connections to the rest of the embryo. The large inner layer of the endosperm that stores nutrients is called the starchy endosperm. The thin outer layer of the endosperm, which is a single layer of cells, is called the aleurone. Upon germination, enzymes are secreted by the aleurone. The enzymes degrade the stored carbohydrates, proteins and lipids, the products of which are absorbed by the scutellum and transported via a vasculature strand to the developing embryo. Therefore, the scutellum can be seen to be an absorptive organ, not a storage organ.
The root tip is protected by a sheath-like structure called the coleorhiza. Similarly, the coleoptile ensheaths the plumule at the shoot tip (Figure \(\PageIndex{7-10}\)).
Other Variations
Seeds are diverse. Pine (Pinus, a gymnosperm and thus neither a monocot nor eudicot) has multiple (five or more) cotyledons. Some plants like orchids (Orchidaceae, a monocot) do not have developed embryo and even endosperm in seeds; their germination depends on a presence of symbiotic (mycorrhizal) fungus.
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/40%3A_Plant_Reproduction/40.05%3A_Embryo_Development/40.5.02%3A_Mature_Embryos_and_Seed_Structure.txt |
Learning Objectives
• Describe the process of double fertilization in plants
Double Fertilization
After pollen is deposited on the stigma, it must germinate and grow through the style to reach the ovule. The microspores, or the pollen, contain two cells: the pollen tube cell and the generative cell. The pollen tube cell grows into a pollen tube through which the generative cell travels. The germination of the pollen tube requires water, oxygen, and certain chemical signals. As it travels through the style to reach the embryo sac, the pollen tube’s growth is supported by the tissues of the style. During this process, if the generative cell has not already split into two cells, it now divides to form two sperm cells. The pollen tube is guided by the chemicals secreted by the synergids present in the embryo sac; it enters the ovule sac through the micropyle. Of the two sperm cells, one sperm fertilizes the egg cell, forming a diploid zygote; the other sperm fuses with the two polar nuclei, forming a triploid cell that develops into the endosperm. Together, these two fertilization events in angiosperms are known as double fertilization. After fertilization is complete, no other sperm can enter. The fertilized ovule forms the seed, whereas the tissues of the ovary become the fruit, usually enveloping the seed.
After fertilization, embryonic development begins. The zygote divides to form two cells: the upper cell (terminal cell) and the lower cell (basal cell). The division of the basal cell gives rise to the suspensor, which eventually makes connection with the maternal tissue. The suspensor provides a route for nutrition to be transported from the mother plant to the growing embryo. The terminal cell also divides, giving rise to a globular-shaped proembryo. In dicots (eudicots), the developing embryo has a heart shape due to the presence of the two rudimentary cotyledons. In non-endospermic dicots, such as Capsella bursa, the endosperm develops initially, but is then digested. In this case, the food reserves are moved into the two cotyledons. As the embryo and cotyledons enlarge, they become crowded inside the developing seed and are forced to bend. Ultimately, the embryo and cotyledons fill the seed, at which point, the seed is ready for dispersal. Embryonic development is suspended after some time; growth resumes only when the seed germinates. The developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.
Key Points
• Double fertilization involves two sperm cells; one fertilizes the egg cell to form the zygote, while the other fuses with the two polar nuclei that form the endosperm.
• After fertilization, the fertilized ovule forms the seed while the tissues of the ovary become the fruit.
• In the first stage of embryonic development, the zygote divides to form two cells; one will develop into a suspensor, while the other gives rise to a proembryo.
• In the second stage of embryonic development (in eudicots), the developing embryo has a heart shape due to the presence of cotyledons.
• As the embryo grows, it begins to bend as it fills the seed; at this point, the seed is ready for dispersal.
Key Terms
• double fertilization: a complex fertilization mechanism that has evolved in flowering plants; involves the joining of a female gametophyte with two male gametes (sperm)
• suspensor: found in plant zygotes in angiosperms; connects the endosperm to the embryo and provides a route for nutrition from the mother plant to the growing embryo
• proembryo: a cluster of cells in the ovule of a fertilized flowering plant that has not yet formed into an embryo | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/40%3A_Plant_Reproduction/40.05%3A_Embryo_Development/40.5D%3A_Double_Fertilization_in_Plants.txt |
Learning Objectives
• Identify the environmental factors that stimulate germination.
• Distinguish between epigeous and hypogeous germination.
• Compare germination in eudicots versus monocots.
Many mature seeds enter a period of inactivity, or extremely low metabolic activity: a process known as dormancy, which may last for months, years or even centuries. Dormancy helps keep seeds viable during unfavorable conditions. Germination occurs when the embryo, which is dormant within a mature seed, resumes growth upon a return to favorable conditions. The embryo becomes a young seedling that is no longer confined within the seed coat.
In many seeds, the presence of a thick seed coat can inhibit germination through several mechanisms: (1) the embryo may not be able to break through the thick seed coat; (2) the seed coat may contain chemicals inhibitors; and (3) the seed coat prevents the embryo from accessing water and oxygen. Dormancy is also maintained by the relative hormone concentrations in the embryo itself.
Environmental Requirements for Germination
The requirements for germination depend on the species. Common environmental requirements include light, the proper temperature, presence of oxygen, and presence of water. Seeds of small-seeded species usually require light as a germination cue. This ensures the seeds only germinate at or near the soil surface (where the light is greatest). If they were to germinate too far underneath the surface, the developing seedling would not have enough food reserves to reach the sunlight. (Recall from 14.5 Dormancy that red light induces germination by converting the inactive form of phytochrome (Pr) to the active form (Pfr), which leads to the production of amylase. This enzyme breaks down the limited food reserves in the seed, facilitating germination.)
Not only do some species require a specific temperature to germinate, but they may also require a prolonged cold period prior to germination. In this case, cold conditions gradually break down a chemical germination inhibitor. This mechanism prevents seeds from germinating during an unseasonably warm spell in the autumn or winter in temperate climates. Similarly, plants growing in hot climates may have seeds that need a hot period in order to germinate, an adaptation to avoid germination in the hot, dry summers.
Water is always needed to allow vigorous metabolism to begin. Additionally, water can leach away inhibitors in the seed coat. This is especially common among desert annuals. Seeds that are dispersed by animals may need to pass through an animal digestive tract to remove inhibitors prior to germination. Similarly, some species require mechanical abrasion of the seed coat, which could be achieved by water dispersal. Other species are fire adapted, requiring fire to break dormancy (Figure \(1\)).
The Mechanism of Germination
The first step in germination and starts with the uptake of water, also known as imbibition. After imbibition, enzymes are activated that start to break down starch into sugars consumed by embryo. The first indication that germination has begun is a swelling in the radicle.
Depending on seed size, the time taken for a seedling to emerge may vary. Species with large seeds have enough food reserves to germinate deep below ground, and still extend their epicotyl all the way to the soil surface while the seedlings of small-seeded species emerge more quickly (and can only germinate close to the surface of the soil).
During epigeous germination, the hypocotyl elongates, and the cotyledons extend above ground. During hypogeous germination, the epicotyl elongates, and the cotyledon(s) remain belowground (Figure \(2\)). Some species (like beans and onions) have epigeous germination while others (like peas and corn) have hypogeous germination. In many epigeous species, the cotyledons not only transfer their food stores to the developing plant but also turn green and make more food by photosynthesis until they drop off.
Germination in Eudicots
Upon germination in eudicot seeds, the radicle emerges from the seed coat while the seed is still buried in the soil.
For epigeous eudicots (like beans), the hypocotyl is shaped like a hook with the plumule pointing downwards. This shape is called the plumule hook, and it persists as long as germination proceeds in the dark. Therefore, as the hypocotyl pushes through the tough and abrasive soil, the plumule is protected from damage. Additionally, the two cotyledons additionally protect the from mechanical damage. Upon exposure to light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl elongates (Figure \(3\)).
In hypogeous eudicots (like peas), the epicotyl rather than the hypocotyl forms a hook, and the cotyledons and hypocotyl thus remain underground. When the epicotyl emerges from the soil, the young foliage leaves expand. The epicotyl continues to elongate (Figure \(4\)).
The radicle continues to grown downwards and ultimately produces the tap root. Lateral roots then branch off to all sides, producing the typical eudicot tap root system.
Germination in Monocots
As the seed germinates, the radicle emerges and forms the first root. In epigeous monocots (such as onion), the single cotyledon will bend, forming a hook and emerge before the coleoptile (Figure \(5\)). In hypogeous monocots (such as corn), the cotyledon remains belowground, and the coleoptile emerges first. In either case, once the coleoptile has exited the soil and is exposed to light, it stops growing. The first leaf of the plumule then pieces the coleoptile (Figure \(6\)), and additional leaves expand and unfold. At the other end of the embryonic axis, the first root soon dies while adventitious roots (roots that arise directly from the shoot system) emerge from the base of the stem (Figure \(7\)). This gives the monocot a fibrous root system.
Attributions
Curated and authored by Melissa Ha using the following sources: | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/40%3A_Plant_Reproduction/40.06%3A_Germination_%28NEW%29/40.6.01%3A_Germination.txt |
Skills to Develop
• Compare the mechanisms and methods of natural and artificial asexual reproduction
• Describe the advantages and disadvantages of natural and artificial asexual reproduction
• Discuss plant life spans
Many plants are able to propagate themselves using asexual reproduction. This method does not require the investment required to produce a flower, attract pollinators, or find a means of seed dispersal. Asexual reproduction produces plants that are genetically identical to the parent plant because no mixing of male and female gametes takes place. Traditionally, these plants survive well under stable environmental conditions when compared with plants produced from sexual reproduction because they carry genes identical to those of their parents.
Many different types of roots exhibit asexual reproduction Figure \(1\). The corm is used by gladiolus and garlic. Bulbs, such as a scaly bulb in lilies and a tunicate bulb in daffodils, are other common examples. A potato is a stem tuber, while parsnip propagates from a taproot. Ginger and iris produce rhizomes, while ivy uses an adventitious root (a root arising from a plant part other than the main or primary root), and the strawberry plant has a stolon, which is also called a runner.
Some plants can produce seeds without fertilization. Either the ovule or part of the ovary, which is diploid in nature, gives rise to a new seed. This method of reproduction is known as apomixis.
An advantage of asexual reproduction is that the resulting plant will reach maturity faster. Since the new plant is arising from an adult plant or plant parts, it will also be sturdier than a seedling. Asexual reproduction can take place by natural or artificial (assisted by humans) means.
Natural Methods of Asexual Reproduction
Natural methods of asexual reproduction include strategies that plants have developed to self-propagate. Many plants—like ginger, onion, gladioli, and dahlia—continue to grow from buds that are present on the surface of the stem. In some plants, such as the sweet potato, adventitious roots or runners can give rise to new plants Figure \(2\). In Bryophyllum and kalanchoe, the leaves have small buds on their margins. When these are detached from the plant, they grow into independent plants; or, they may start growing into independent plants if the leaf touches the soil. Some plants can be propagated through cuttings alone.
Artificial Methods of Asexual Reproduction
These methods are frequently employed to give rise to new, and sometimes novel, plants. They include grafting, cutting, layering, and micropropagation.
Grafting
Grafting has long been used to produce novel varieties of roses, citrus species, and other plants. In grafting, two plant species are used; part of the stem of the desirable plant is grafted onto a rooted plant called the stock. The part that is grafted or attached is called the scion. Both are cut at an oblique angle (any angle other than a right angle), placed in close contact with each other, and are then held together Figure \(3\). Matching up these two surfaces as closely as possible is extremely important because these will be holding the plant together. The vascular systems of the two plants grow and fuse, forming a graft. After a period of time, the scion starts producing shoots, and eventually starts bearing flowers and fruits. Grafting is widely used in viticulture (grape growing) and the citrus industry. Scions capable of producing a particular fruit variety are grated onto root stock with specific resistance to disease.
Cutting
Plants such as coleus and money plant are propagated through stem cuttings, where a portion of the stem containing nodes and internodes is placed in moist soil and allowed to root. In some species, stems can start producing a root even when placed only in water. For example, leaves of the African violet will root if kept in water undisturbed for several weeks.
Layering
Layering is a method in which a stem attached to the plant is bent and covered with soil. Young stems that can be bent easily without any injury are preferred. Jasmine and bougainvillea (paper flower) can be propagated this way Figure \(4\). In some plants, a modified form of layering known as air layering is employed. A portion of the bark or outermost covering of the stem is removed and covered with moss, which is then taped. Some gardeners also apply rooting hormone. After some time, roots will appear, and this portion of the plant can be removed and transplanted into a separate pot.
Micropropagation
Micropropagation (also called plant tissue culture) is a method of propagating a large number of plants from a single plant in a short time under laboratory conditions Figure \(5\). This method allows propagation of rare, endangered species that may be difficult to grow under natural conditions, are economically important, or are in demand as disease-free plants.
To start plant tissue culture, a part of the plant such as a stem, leaf, embryo, anther, or seed can be used. The plant material is thoroughly sterilized using a combination of chemical treatments standardized for that species. Under sterile conditions, the plant material is placed on a plant tissue culture medium that contains all the minerals, vitamins, and hormones required by the plant. The plant part often gives rise to an undifferentiated mass known as callus, from which individual plantlets begin to grow after a period of time. These can be separated and are first grown under greenhouse conditions before they are moved to field conditions.
Plant Life Spans
The length of time from the beginning of development to the death of a plant is called its life span. The life cycle, on the other hand, is the sequence of stages a plant goes through from seed germination to seed production of the mature plant. Some plants, such as annuals, only need a few weeks to grow, produce seeds and die. Other plants, such as the bristlecone pine, live for thousands of years. Some bristlecone pines have a documented age of 4,500 years Figure \(6\). Even as some parts of a plant, such as regions containing meristematic tissue—the area of active plant growth consisting of undifferentiated cells capable of cell division—continue to grow, some parts undergo programmed cell death (apoptosis). The cork found on stems, and the water-conducting tissue of the xylem, for example, are composed of dead cells.
Plant species that complete their lifecycle in one season are known as annuals, an example of which is Arabidopsis, or mouse-ear cress. Biennials such as carrots complete their lifecycle in two seasons. In a biennial’s first season, the plant has a vegetative phase, whereas in the next season, it completes its reproductive phase. Commercial growers harvest the carrot roots after the first year of growth, and do not allow the plants to flower. Perennials, such as the magnolia, complete their lifecycle in two years or more.
In another classification based on flowering frequency, monocarpic plants flower only once in their lifetime; examples include bamboo and yucca. During the vegetative period of their life cycle (which may be as long as 120 years in some bamboo species), these plants may reproduce asexually and accumulate a great deal of food material that will be required during their once-in-a-lifetime flowering and setting of seed after fertilization. Soon after flowering, these plants die. Polycarpic plants form flowers many times during their lifetime. Fruit trees, such as apple and orange trees, are polycarpic; they flower every year. Other polycarpic species, such as perennials, flower several times during their life span, but not each year. By this means, the plant does not require all its nutrients to be channelled towards flowering each year.
As is the case with all living organisms, genetics and environmental conditions have a role to play in determining how long a plant will live. Susceptibility to disease, changing environmental conditions, drought, cold, and competition for nutrients are some of the factors that determine the survival of a plant. Plants continue to grow, despite the presence of dead tissue such as cork. Individual parts of plants, such as flowers and leaves, have different rates of survival. In many trees, the older leaves turn yellow and eventually fall from the tree. Leaf fall is triggered by factors such as a decrease in photosynthetic efficiency, due to shading by upper leaves, or oxidative damage incurred as a result of photosynthetic reactions. The components of the part to be shed are recycled by the plant for use in other processes, such as development of seed and storage. This process is known as nutrient recycling.
The aging of a plant and all the associated processes is known as senescence, which is marked by several complex biochemical changes. One of the characteristics of senescence is the breakdown of chloroplasts, which is characterized by the yellowing of leaves. The chloroplasts contain components of photosynthetic machinery such as membranes and proteins. Chloroplasts also contain DNA. The proteins, lipids, and nucleic acids are broken down by specific enzymes into smaller molecules and salvaged by the plant to support the growth of other plant tissues.
The complex pathways of nutrient recycling within a plant are not well understood. Hormones are known to play a role in senescence. Applications of cytokinins and ethylene delay or prevent senescence; in contrast, abscissic acid causes premature onset of senescence.
Summary
Many plants reproduce asexually as well as sexually. In asexual reproduction, part of the parent plant is used to generate a new plant. Grafting, layering, and micropropagation are some methods used for artificial asexual reproduction. The new plant is genetically identical to the parent plant from which the stock has been taken. Asexually reproducing plants thrive well in stable environments.
Plants have different life spans, dependent on species, genotype, and environmental conditions. Parts of the plant, such as regions containing meristematic tissue, continue to grow, while other parts experience programmed cell death. Leaves that are no longer photosynthetically active are shed from the plant as part of senescence, and the nutrients from these leaves are recycled by the plant. Other factors, including the presence of hormones, are known to play a role in delaying senescence.
Glossary
apomixis
process by which seeds are produced without fertilization of sperm and egg
cutting
method of asexual reproduction where a portion of the stem contains notes and internodes is placed in moist soil and allowed to root
grafting
method of asexual reproduction where the stem from one plant species is spliced to a different plant
layering
method of propagating plants by bending a stem under the soil
micropropagation
propagation of desirable plants from a plant part; carried out in a laboratory
monocarpic
plants that flower once in their lifetime
polycarpic
plants that flower several times in their lifetime
scion
the part of a plant that is grafted onto the root stock of another plant
senescence
process that describes aging in plant tissues | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/40%3A_Plant_Reproduction/40.07%3A_Asexual_Reproduction.txt |
Learning Objectives
• Explain the process of aging in plants
Plant Life Spans
The length of time from the beginning of development to the death of a plant is called its life span. The life cycle, on the other hand, is the sequence of stages a plant goes through from seed germination to seed production of the mature plant. Some plants, such as annuals, only need a few weeks to grow, produce seeds, and die. Other plants, such as the bristlecone pine, live for thousands of years. Some bristlecone pines have a documented age of 4,500 years. Even as some parts of a plant, such as regions containing meristematic tissue (the area of active plant growth consisting of undifferentiated cells capable of cell division) continue to grow, some parts undergo programmed cell death (apoptosis). The cork found on stems and the water-conducting tissue of the xylem, for example, are composed of dead cells.
Annuals, Biennial, and Perennials
Plant species that complete their life cycle in one season are known as annuals, an example of which is Arabidopsis, or mouse-ear cress. Biennials, such as carrots, complete their life cycle in two seasons. In a biennial’s first season, the plant has a vegetative phase, whereas in the next season, it completes its reproductive phase. Commercial growers harvest the carrot roots after the first year of growth and do not allow the plants to flower. Perennials, such as the magnolia, complete their life cycle in two years or more.
Monocarpic and Polycarpic Plants
In another classification based on flowering frequency, monocarpic plants flower only once in their lifetime; examples of monocarpic plants include bamboo and yucca. During the vegetative period of their life cycle (which may be as long as 120 years in some bamboo species), these plants may reproduce asexually, accumulating a great deal of food material that will be required during their once-in-a-lifetime flowering and setting of seed after fertilization. Soon after flowering, these plants die. Polycarpic plants form flowers many times during their lifetime. Fruit trees, such as apple and orange trees, are polycarpic; they flower every year. Other polycarpic species, such as perennials, flower several times during their life span, but not each year. By this method, the plant does not require all its nutrients to be channeled towards flowering each year.
Genetics and Environmental Conditions
As is the case with all living organisms, genetics and environmental conditions have a role to play in determining how long a plant will live. Susceptibility to disease, changing environmental conditions, drought, cold, and competition for nutrients are some of the factors that determine the survival of a plant. Plants continue to grow, despite the presence of dead tissue, such as cork. Individual parts of plants, such as flowers and leaves, have different rates of survival. In many trees, the older leaves turn yellow and eventually fall from the tree. Leaf fall is triggered by factors such as a decrease in photosynthetic efficiency due to shading by upper leaves or oxidative damage incurred as a result of photosynthetic reactions. The components of the part to be shed are recycled by the plant for use in other processes, such as development of seed and storage. This process is known as nutrient recycling. However, the complex pathways of nutrient recycling within a plant are not well understood
The aging of a plant and all the associated processes is known as senescence, which is marked by several complex biochemical changes. One of the characteristics of senescence is the breakdown of chloroplasts, which is characterized by the yellowing of leaves. The chloroplasts contain components of photosynthetic machinery, such as membranes and proteins. Chloroplasts also contain DNA. The proteins, lipids, and nucleic acids are broken down by specific enzymes into smaller molecules and salvaged by the plant to support the growth of other plant tissues. Hormones are known to play a role in senescence. Applications of cytokinins and ethylene delay or prevent senescence; in contrast, abscissic acid causes premature onset of senescence.
Key Points
• The life span of a plant is the length of time it takes from the beginning of development until death, while the life cycle is the series of stages between the germination of the seed until the plant produces its own seeds.
• Annuals complete their life cycle in one season; biennials complete their life cycle in two seasons; and perennials complete their life cycle in more than two seasons.
• Monocarpic plants flower only once in their lifetime, while polycarpic plants flower more than once.
• Plant survival depends on changing environmental conditions, drought, cold, and competition.
• Senescence refers to aging of the plant, during which components of the plant cells are broken down and used to support the growth of other plant tissues.
Key Terms
• annual: a plant which naturally germinates, flowers, and dies in one year
• biennial: a plant that requires two years to complete its life cycle
• perennial: a plant that is active throughout the year or survives for more than two growing seasons
• monocarpic: a plant that flowers and bears fruit only once before dying
• polycarpic: bearing fruit repeatedly, or year after year
• senescence: aging of a plant; accumulated damage to macromolecules, cells, tissues, and organs with the passage of time | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/40%3A_Plant_Reproduction/40.08%3A_Plant_Life_Spans.txt |
• 41.1: Organization of Animal Bodies
The tissues of multicellular, complex animals are four primary types: epithelial, connective, muscle, and nervous. Recall that tissues are groups of similar cells group of similar cells carrying out related functions. These tissues combine to form organs—like the skin or kidney—that have specific, specialized functions within the body. Organs are organized into organ systems to perform functions.
• 41.2: Epithelial Tissue
The tissues of multicellular, complex animals are four primary types: epithelial, connective, muscle, and nervous. Recall that tissues are groups of similar cells group of similar cells carrying out related functions. These tissues combine to form organs—like the skin or kidney—that have specific, specialized functions within the body. Organs are organized into organ systems to perform functions.
• 41.3: Connective Tissue
The tissues of multicellular, complex animals are four primary types: epithelial, connective, muscle, and nervous. Recall that tissues are groups of similar cells group of similar cells carrying out related functions. These tissues combine to form organs—like the skin or kidney—that have specific, specialized functions within the body. Organs are organized into organ systems to perform functions.
• 41.4: Muscle Tissue
The tissues of multicellular, complex animals are four primary types: epithelial, connective, muscle, and nervous. Recall that tissues are groups of similar cells group of similar cells carrying out related functions. These tissues combine to form organs—like the skin or kidney—that have specific, specialized functions within the body. Organs are organized into organ systems to perform functions.
• 41.5: Nerve Tissue
Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in Figure 35.1.1. Some organisms, like sea sponges, lack a true nervous system. Others, like jellyfish, lack a true brain and instead have a system of separate but connected nerve cells (neurons) called a “nerve net.” Echinoderms such as sea stars have nerve cells that are bundled into fibers called nerves.
• 41.6: Overview of Vertebrate Organ Systems
• 41.7: Homeostasis
Animal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis (“steady state”). These changes might be in the level of glucose or calcium in blood or in external temperatures. Homeostasis means to maintain dynamic equilibrium in the body. It is dynamic because it is constantly adjusting to the changes that the body’s systems encounter. It is equilibrium because body functions are kept within specific ranges.
• 41.8: Regulating Body Temperature
Animals vary in form and function. From a sponge to a worm to a goat, an organism has a distinct body plan that limits its size and shape. Animals’ bodies are also designed to interact with their environments, whether in the deep sea, a rainforest canopy, or the desert. Therefore, a large amount of information about the structure of an organism's body (anatomy) and the function of its cells, tissues and organs (physiology) can be learned by studying that organism's environment.
41: The Animal Body and Principles of Regulation
Skills to Develop
• Describe epithelial tissues
• Discuss the different types of connective tissues in animals
• Describe three types of muscle tissues
• Describe nervous tissue
The tissues of multicellular, complex animals are four primary types: epithelial, connective, muscle, and nervous. Recall that tissues are groups of similar cells group of similar cells carrying out related functions. These tissues combine to form organs—like the skin or kidney—that have specific, specialized functions within the body. Organs are organized into organ systems to perform functions; examples include the circulatory system, which consists of the heart and blood vessels, and the digestive system, consisting of several organs, including the stomach, intestines, liver, and pancreas. Organ systems come together to create an entire organism.
Epithelial Tissues
Epithelial tissues cover the outside of organs and structures in the body and line the lumens of organs in a single layer or multiple layers of cells. The types of epithelia are classified by the shapes of cells present and the number of layers of cells. Epithelia composed of a single layer of cells is called simple epithelia; epithelial tissue composed of multiple layers is called stratified epithelia. The table summarizes the different types of epithelial tissues.
Table \(1\): Different Types of Epithelial Tissues
Cell shape Description Location
squamous flat, irregular round shape simple: lung alveoli, capillaries stratified: skin, mouth, vagina
cuboidal cube shaped, central nucleus glands, renal tubules
columnar tall, narrow, nucleus toward base tall, narrow, nucleus along cell simple: digestive tract pseudostratified: respiratory tract
transitional round, simple but appear stratified urinary bladder
Squamous Epithelia
Squamous epithelial cells are generally round, flat, and have a small, centrally located nucleus. The cell outline is slightly irregular, and cells fit together to form a covering or lining. When the cells are arranged in a single layer (simple epithelia), they facilitate diffusion in tissues, such as the areas of gas exchange in the lungs and the exchange of nutrients and waste at blood capillaries.
Figure \(1\) illustrates a layer of squamous cells with their membranes joined together to form an epithelium. Image Figure \(1\) illustrates squamous epithelial cells arranged in stratified layers, where protection is needed on the body from outside abrasion and damage. This is called a stratified squamous epithelium and occurs in the skin and in tissues lining the mouth and vagina.
Cuboidal Epithelia
Cuboidal epithelial cells, shown in Figure \(2\), are cube-shaped with a single, central nucleus. They are most commonly found in a single layer representing a simple epithelia in glandular tissues throughout the body where they prepare and secrete glandular material. They are also found in the walls of tubules and in the ducts of the kidney and liver.
Columnar Epithelia
Columnar epithelial cells are taller than they are wide: they resemble a stack of columns in an epithelial layer, and are most commonly found in a single-layer arrangement. The nuclei of columnar epithelial cells in the digestive tract appear to be lined up at the base of the cells, as illustrated in Figure \(3\). These cells absorb material from the lumen of the digestive tract and prepare it for entry into the body through the circulatory and lymphatic systems.
Columnar epithelial cells lining the respiratory tract appear to be stratified. However, each cell is attached to the base membrane of the tissue and, therefore, they are simple tissues. The nuclei are arranged at different levels in the layer of cells, making it appear as though there is more than one layer, as seen in Figure \(4\). This is called pseudostratified, columnar epithelia. This cellular covering has cilia at the apical, or free, surface of the cells. The cilia enhance the movement of mucous and trapped particles out of the respiratory tract, helping to protect the system from invasive microorganisms and harmful material that has been breathed into the body. Goblet cells are interspersed in some tissues (such as the lining of the trachea). The goblet cells contain mucous that traps irritants, which in the case of the trachea keep these irritants from getting into the lungs.
Transitional Epithelia
Transitional or uroepithelial cells appear only in the urinary system, primarily in the bladder and ureter. These cells are arranged in a stratified layer, but they have the capability of appearing to pile up on top of each other in a relaxed, empty bladder, as illustrated in Figure \(5\). As the urinary bladder fills, the epithelial layer unfolds and expands to hold the volume of urine introduced into it. As the bladder fills, it expands and the lining becomes thinner. In other words, the tissue transitions from thick to thin.
Exercise
Which of the following statements about types of epithelial cells is false?
1. Simple columnar epithelial cells line the tissue of the lung.
2. Simple cuboidal epithelial cells are involved in the filtering of blood in the kidney.
3. Pseudostratisfied columnar epithilia occur in a single layer, but the arrangement of nuclei makes it appear that more than one layer is present.
4. Transitional epithelia change in thickness depending on how full the bladder is.
Answer
A
Connective Tissues
Connective tissues are made up of a matrix consisting of living cells and a non-living substance, called the ground substance. The ground substance is made of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principal cell of connective tissues is the fibroblast. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and, occasionally, leukocytes can be found in some of the tissues. Some tissues have specialized cells that are not found in the others. The matrix in connective tissues gives the tissue its density. When a connective tissue has a high concentration of cells or fibers, it has proportionally a less dense matrix.
The organic portion or protein fibers found in connective tissues are either collagen, elastic, or reticular fibers. Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding tissues. Elastic fibers are made of the protein elastin; this fiber can stretch to one and one half of its length and return to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers are the third type of protein fiber found in connective tissues. This fiber consists of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected. The various types of connective tissues, the types of cells and fibers they are made of, and sample locations of the tissues is summarized in the table.
Table \(2\): Connective Tissues
Tissue Cells Fibers Location
loose/areolar fibroblasts, macrophages, some lymphocytes, some neutrophils few: collagen, elastic, reticular around blood vessels; anchors epithelia
dense, fibrous connective tissue fibroblasts, macrophages, mostly collagen irregular: skin regular: tendons, ligaments
cartilage chondrocytes, chondroblasts hyaline: few collagen fibrocartilage: large amount of collagen shark skeleton, fetal bones, human ears, intervertebral discs
bone osteoblasts, osteocytes, osteoclasts some: collagen, elastic vertebrate skeletons
adipose adipocytes few adipose (fat)
blood red blood cells, white blood cells none blood
Loose/Areolar Connective Tissue
Loose connective tissue, also called areolar connective tissue, has a sampling of all of the components of a connective tissue. As illustrated in Figure \(6\), loose connective tissue has some fibroblasts; macrophages are present as well. Collagen fibers are relatively wide and stain a light pink, while elastic fibers are thin and stain dark blue to black. The space between the formed elements of the tissue is filled with the matrix. The material in the connective tissue gives it a loose consistency similar to a cotton ball that has been pulled apart. Loose connective tissue is found around every blood vessel and helps to keep the vessel in place. The tissue is also found around and between most body organs. In summary, areolar tissue is tough, yet flexible, and comprises membranes.
Fibrous Connective Tissue
Fibrous connective tissues contain large amounts of collagen fibers and few cells or matrix material. The fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly arranged fibrous connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of the skin. Regular fibrous connective tissue, shown in Figure \(7\), is found in tendons (which connect muscles to bones) and ligaments (which connect bones to bones).
Cartilage
Cartilage is a connective tissue with a large amount of the matrix and variable amounts of fibers. The cells, called chondrocytes, make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called lacunae.
A cartilage with few collagen and elastic fibers is hyaline cartilage, illustrated in Figure \(8\). The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine histological stains. Sharks have cartilaginous skeletons, as does nearly the entire human skeleton during a specific pre-birth developmental stage. A remnant of this cartilage persists in the outer portion of the human nose. Hyaline cartilage is also found at the ends of long bones, reducing friction and cushioning the articulations of these bones.
Elastic cartilage has a large amount of elastic fibers, giving it tremendous flexibility. The ears of most vertebrate animals contain this cartilage as do portions of the larynx, or voice box. Fibrocartilage contains a large amount of collagen fibers, giving the tissue tremendous strength. Fibrocartilage comprises the intervertebral discs in vertebrate animals. Hyaline cartilage found in movable joints such as the knee and shoulder becomes damaged as a result of age or trauma. Damaged hyaline cartilage is replaced by fibrocartilage and results in the joints becoming “stiff.”
Bone
Bone, or osseous tissue, is a connective tissue that has a large amount of two different types of matrix material. The organic matrix is similar to the matrix material found in other connective tissues, including some amount of collagen and elastic fibers. This gives strength and flexibility to the tissue. The inorganic matrix consists of mineral salts—mostly calcium salts—that give the tissue hardness. Without adequate organic material in the matrix, the tissue breaks; without adequate inorganic material in the matrix, the tissue bends.
There are three types of cells in bone: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are active in making bone for growth and remodeling. Osteoblasts deposit bone material into the matrix and, after the matrix surrounds them, they continue to live, but in a reduced metabolic state as osteocytes. Osteocytes are found in lacunae of the bone. Osteoclasts are active in breaking down bone for bone remodeling, and they provide access to calcium stored in tissues. Osteoclasts are usually found on the surface of the tissue.
Bone can be divided into two types: compact and spongy. Compact bone is found in the shaft (or diaphysis) of a long bone and the surface of the flat bones, while spongy bone is found in the end (or epiphysis) of a long bone. Compact bone is organized into subunits called osteons, as illustrated in Figure \(9\). A blood vessel and a nerve are found in the center of the structure within the Haversian canal, with radiating circles of lacunae around it known as lamellae. The wavy lines seen between the lacunae are microchannels called canaliculi; they connect the lacunae to aid diffusion between the cells. Spongy bone is made of tiny plates called trabeculae these plates serve as struts to give the spongy bone strength. Over time, these plates can break causing the bone to become less resilient. Bone tissue forms the internal skeleton of vertebrate animals, providing structure to the animal and points of attachment for tendons.
Adipose Tissue
Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a real matrix and only has a few fibers. Adipose tissue is made up of cells called adipocytes that collect and store fat in the form of triglycerides, for energy metabolism. Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic, and they function as cushioning against damage to body organs. Under a microscope, adipose tissue cells appear empty due to the extraction of fat during the processing of the material for viewing, as seen in Figure \(10\). The thin lines in the image are the cell membranes, and the nuclei are the small, black dots at the edges of the cells.
Blood
Blood is considered a connective tissue because it has a matrix, as shown in Figure \(11\). The living cell types are red blood cells (RBC), also called erythrocytes, and white blood cells (WBC), also called leukocytes. The fluid portion of whole blood, its matrix, is commonly called plasma.
The cell found in greatest abundance in blood is the erythrocyte. Erythrocytes are counted in millions in a blood sample: the average number of red blood cells in primates is 4.7 to 5.5 million cells per microliter. Erythrocytes are consistently the same size in a species, but vary in size between species. For example, the average diameter of a primate red blood cell is 7.5 µl, a dog is close at 7.0 µl, but a cat’s RBC diameter is 5.9 µl. Sheep erythrocytes are even smaller at 4.6 µl. Mammalian erythrocytes lose their nuclei and mitochondria when they are released from the bone marrow where they are made. Fish, amphibian, and avian red blood cells maintain their nuclei and mitochondria throughout the cell’s life. The principal job of an erythrocyte is to carry and deliver oxygen to the tissues.
Leukocytes are the predominant white blood cells found in the peripheral blood. Leukocytes are counted in the thousands in the blood with measurements expressed as ranges: primate counts range from 4,800 to 10,800 cells per µl, dogs from 5,600 to 19,200 cells per µl, cats from 8,000 to 25,000 cells per µl, cattle from 4,000 to 12,000 cells per µl, and pigs from 11,000 to 22,000 cells per µl.
Lymphocytes function primarily in the immune response to foreign antigens or material. Different types of lymphocytes make antibodies tailored to the foreign antigens and control the production of those antibodies. Neutrophils are phagocytic cells and they participate in one of the early lines of defense against microbial invaders, aiding in the removal of bacteria that has entered the body. Another leukocyte that is found in the peripheral blood is the monocyte. Monocytes give rise to phagocytic macrophages that clean up dead and damaged cells in the body, whether they are foreign or from the host animal. Two additional leukocytes in the blood are eosinophils and basophils—both help to facilitate the inflammatory response.
The slightly granular material among the cells is a cytoplasmic fragment of a cell in the bone marrow. This is called a platelet or thrombocyte. Platelets participate in the stages leading up to coagulation of the blood to stop bleeding through damaged blood vessels. Blood has a number of functions, but primarily it transports material through the body to bring nutrients to cells and remove waste material from them.
Muscle Tissues
There are three types of muscle in animal bodies: smooth, skeletal, and cardiac. They differ by the presence or absence of striations or bands, the number and location of nuclei, whether they are voluntarily or involuntarily controlled, and their location within the body. The table summarizes these differences.
Table \(3\): Types of Muscles
Type of Muscle Striations Nuclei Control Location
smooth no single, in center involuntary visceral organs
skeletal yes many, at periphery voluntary skeletal muscles
cardiac yes single, in center involuntary heart
Smooth Muscle
Smooth muscle does not have striations in its cells. It has a single, centrally located nucleus, as shown in Figure \(12\). Constriction of smooth muscle occurs under involuntary, autonomic nervous control and in response to local conditions in the tissues. Smooth muscle tissue is also called non-striated as it lacks the banded appearance of skeletal and cardiac muscle. The walls of blood vessels, the tubes of the digestive system, and the tubes of the reproductive systems are composed of mostly smooth muscle.
Skeletal Muscle
Skeletal muscle has striations across its cells caused by the arrangement of the contractile proteins actin and myosin. These muscle cells are relatively long and have multiple nuclei along the edge of the cell. Skeletal muscle is under voluntary, somatic nervous system control and is found in the muscles that move bones. Figure \(12\)illustrates the histology of skeletal muscle.
Cardiac Muscle
Cardiac muscle, shown in Figure \(12\), is found only in the heart. Like skeletal muscle, it has cross striations in its cells, but cardiac muscle has a single, centrally located nucleus. Cardiac muscle is not under voluntary control but can be influenced by the autonomic nervous system to speed up or slow down. An added feature to cardiac muscle cells is a line than extends along the end of the cell as it abuts the next cardiac cell in the row. This line is called an intercalated disc: it assists in passing electrical impulse efficiently from one cell to the next and maintains the strong connection between neighboring cardiac cells.
Nervous Tissues
Nervous tissues are made of cells specialized to receive and transmit electrical impulses from specific areas of the body and to send them to specific locations in the body. The main cell of the nervous system is the neuron, illustrated in Figure \(13\). The large structure with a central nucleus is the cell body of the neuron. Projections from the cell body are either dendrites specialized in receiving input or a single axon specialized in transmitting impulses. Some glial cells are also shown. Astrocytes regulate the chemical environment of the nerve cell, and oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently. Other glial cells that are not shown support the nutritional and waste requirements of the neuron. Some of the glial cells are phagocytic and remove debris or damaged cells from the tissue. A nerve consists of neurons and glial cells.
Link to Learning
Click through the interactive review to learn more about epithelial tissues.
Career Connections: Pathologist
A pathologist is a medical doctor or veterinarian who has specialized in the laboratory detection of disease in animals, including humans. These professionals complete medical school education and follow it with an extensive post-graduate residency at a medical center. A pathologist may oversee clinical laboratories for the evaluation of body tissue and blood samples for the detection of disease or infection. They examine tissue specimens through a microscope to identify cancers and other diseases. Some pathologists perform autopsies to determine the cause of death and the progression of disease.
Summary
The basic building blocks of complex animals are four primary tissues. These are combined to form organs, which have a specific, specialized function within the body, such as the skin or kidney. Organs are organized together to perform common functions in the form of systems. The four primary tissues are epithelia, connective tissues, muscle tissues, and nervous tissues.
Glossary
canaliculus
microchannel that connects the lacunae and aids diffusion between cells
cartilage
type of connective tissue with a large amount of ground substance matrix, cells called chondrocytes, and some amount of fibers
chondrocyte
cell found in cartilage
columnar epithelia
epithelia made of cells taller than they are wide, specialized in absorption
connective tissue
type of tissue made of cells, ground substance matrix, and fibers
cuboidal epithelia
epithelia made of cube-shaped cells, specialized in glandular functions
epithelial tissue
tissue that either lines or covers organs or other tissues
fibrous connective tissue
type of connective tissue with a high concentration of fibers
lacuna
space in cartilage and bone that contains living cells
loose (areolar) connective tissue
type of connective tissue with small amounts of cells, matrix, and fibers; found around blood vessels
matrix
component of connective tissue made of both living and non-living (ground substances) cells
osteon
subunit of compact bone
pseudostratified
layer of epithelia that appears multilayered, but is a simple covering
simple epithelia
single layer of epithelial cells
squamous epithelia
type of epithelia made of flat cells, specialized in aiding diffusion or preventing abrasion
stratified epithelia
multiple layers of epithelial cells
trabecula
tiny plate that makes up spongy bone and gives it strength
transitional epithelia
epithelia that can transition for appearing multilayered to simple; also called uroepithelial | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/41%3A_The_Animal_Body_and_Principles_of_Regulation/41.01%3A_Organization_of_Animal_Bodies.txt |
Skills to Develop
• Describe epithelial tissues
• Discuss the different types of connective tissues in animals
• Describe three types of muscle tissues
• Describe nervous tissue
The tissues of multicellular, complex animals are four primary types: epithelial, connective, muscle, and nervous. Recall that tissues are groups of similar cells group of similar cells carrying out related functions. These tissues combine to form organs—like the skin or kidney—that have specific, specialized functions within the body. Organs are organized into organ systems to perform functions; examples include the circulatory system, which consists of the heart and blood vessels, and the digestive system, consisting of several organs, including the stomach, intestines, liver, and pancreas. Organ systems come together to create an entire organism.
Epithelial Tissues
Epithelial tissues cover the outside of organs and structures in the body and line the lumens of organs in a single layer or multiple layers of cells. The types of epithelia are classified by the shapes of cells present and the number of layers of cells. Epithelia composed of a single layer of cells is called simple epithelia; epithelial tissue composed of multiple layers is called stratified epithelia. The table summarizes the different types of epithelial tissues.
Table \(1\): Different Types of Epithelial Tissues
Cell shape Description Location
squamous flat, irregular round shape simple: lung alveoli, capillaries stratified: skin, mouth, vagina
cuboidal cube shaped, central nucleus glands, renal tubules
columnar tall, narrow, nucleus toward base tall, narrow, nucleus along cell simple: digestive tract pseudostratified: respiratory tract
transitional round, simple but appear stratified urinary bladder
Squamous Epithelia
Squamous epithelial cells are generally round, flat, and have a small, centrally located nucleus. The cell outline is slightly irregular, and cells fit together to form a covering or lining. When the cells are arranged in a single layer (simple epithelia), they facilitate diffusion in tissues, such as the areas of gas exchange in the lungs and the exchange of nutrients and waste at blood capillaries.
Figure \(1\) illustrates a layer of squamous cells with their membranes joined together to form an epithelium. Image Figure \(1\) illustrates squamous epithelial cells arranged in stratified layers, where protection is needed on the body from outside abrasion and damage. This is called a stratified squamous epithelium and occurs in the skin and in tissues lining the mouth and vagina.
Cuboidal Epithelia
Cuboidal epithelial cells, shown in Figure \(2\), are cube-shaped with a single, central nucleus. They are most commonly found in a single layer representing a simple epithelia in glandular tissues throughout the body where they prepare and secrete glandular material. They are also found in the walls of tubules and in the ducts of the kidney and liver.
Columnar Epithelia
Columnar epithelial cells are taller than they are wide: they resemble a stack of columns in an epithelial layer, and are most commonly found in a single-layer arrangement. The nuclei of columnar epithelial cells in the digestive tract appear to be lined up at the base of the cells, as illustrated in Figure \(3\). These cells absorb material from the lumen of the digestive tract and prepare it for entry into the body through the circulatory and lymphatic systems.
Columnar epithelial cells lining the respiratory tract appear to be stratified. However, each cell is attached to the base membrane of the tissue and, therefore, they are simple tissues. The nuclei are arranged at different levels in the layer of cells, making it appear as though there is more than one layer, as seen in Figure \(4\). This is called pseudostratified, columnar epithelia. This cellular covering has cilia at the apical, or free, surface of the cells. The cilia enhance the movement of mucous and trapped particles out of the respiratory tract, helping to protect the system from invasive microorganisms and harmful material that has been breathed into the body. Goblet cells are interspersed in some tissues (such as the lining of the trachea). The goblet cells contain mucous that traps irritants, which in the case of the trachea keep these irritants from getting into the lungs.
Transitional Epithelia
Transitional or uroepithelial cells appear only in the urinary system, primarily in the bladder and ureter. These cells are arranged in a stratified layer, but they have the capability of appearing to pile up on top of each other in a relaxed, empty bladder, as illustrated in Figure \(5\). As the urinary bladder fills, the epithelial layer unfolds and expands to hold the volume of urine introduced into it. As the bladder fills, it expands and the lining becomes thinner. In other words, the tissue transitions from thick to thin.
Exercise
Which of the following statements about types of epithelial cells is false?
1. Simple columnar epithelial cells line the tissue of the lung.
2. Simple cuboidal epithelial cells are involved in the filtering of blood in the kidney.
3. Pseudostratisfied columnar epithilia occur in a single layer, but the arrangement of nuclei makes it appear that more than one layer is present.
4. Transitional epithelia change in thickness depending on how full the bladder is.
Answer
A
Connective Tissues
Connective tissues are made up of a matrix consisting of living cells and a non-living substance, called the ground substance. The ground substance is made of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principal cell of connective tissues is the fibroblast. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and, occasionally, leukocytes can be found in some of the tissues. Some tissues have specialized cells that are not found in the others. The matrix in connective tissues gives the tissue its density. When a connective tissue has a high concentration of cells or fibers, it has proportionally a less dense matrix.
The organic portion or protein fibers found in connective tissues are either collagen, elastic, or reticular fibers. Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding tissues. Elastic fibers are made of the protein elastin; this fiber can stretch to one and one half of its length and return to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers are the third type of protein fiber found in connective tissues. This fiber consists of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected. The various types of connective tissues, the types of cells and fibers they are made of, and sample locations of the tissues is summarized in the table.
Table \(2\): Connective Tissues
Tissue Cells Fibers Location
loose/areolar fibroblasts, macrophages, some lymphocytes, some neutrophils few: collagen, elastic, reticular around blood vessels; anchors epithelia
dense, fibrous connective tissue fibroblasts, macrophages, mostly collagen irregular: skin regular: tendons, ligaments
cartilage chondrocytes, chondroblasts hyaline: few collagen fibrocartilage: large amount of collagen shark skeleton, fetal bones, human ears, intervertebral discs
bone osteoblasts, osteocytes, osteoclasts some: collagen, elastic vertebrate skeletons
adipose adipocytes few adipose (fat)
blood red blood cells, white blood cells none blood
Loose/Areolar Connective Tissue
Loose connective tissue, also called areolar connective tissue, has a sampling of all of the components of a connective tissue. As illustrated in Figure \(6\), loose connective tissue has some fibroblasts; macrophages are present as well. Collagen fibers are relatively wide and stain a light pink, while elastic fibers are thin and stain dark blue to black. The space between the formed elements of the tissue is filled with the matrix. The material in the connective tissue gives it a loose consistency similar to a cotton ball that has been pulled apart. Loose connective tissue is found around every blood vessel and helps to keep the vessel in place. The tissue is also found around and between most body organs. In summary, areolar tissue is tough, yet flexible, and comprises membranes.
Fibrous Connective Tissue
Fibrous connective tissues contain large amounts of collagen fibers and few cells or matrix material. The fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly arranged fibrous connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of the skin. Regular fibrous connective tissue, shown in Figure \(7\), is found in tendons (which connect muscles to bones) and ligaments (which connect bones to bones).
Cartilage
Cartilage is a connective tissue with a large amount of the matrix and variable amounts of fibers. The cells, called chondrocytes, make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called lacunae.
A cartilage with few collagen and elastic fibers is hyaline cartilage, illustrated in Figure \(8\). The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine histological stains. Sharks have cartilaginous skeletons, as does nearly the entire human skeleton during a specific pre-birth developmental stage. A remnant of this cartilage persists in the outer portion of the human nose. Hyaline cartilage is also found at the ends of long bones, reducing friction and cushioning the articulations of these bones.
Elastic cartilage has a large amount of elastic fibers, giving it tremendous flexibility. The ears of most vertebrate animals contain this cartilage as do portions of the larynx, or voice box. Fibrocartilage contains a large amount of collagen fibers, giving the tissue tremendous strength. Fibrocartilage comprises the intervertebral discs in vertebrate animals. Hyaline cartilage found in movable joints such as the knee and shoulder becomes damaged as a result of age or trauma. Damaged hyaline cartilage is replaced by fibrocartilage and results in the joints becoming “stiff.”
Bone
Bone, or osseous tissue, is a connective tissue that has a large amount of two different types of matrix material. The organic matrix is similar to the matrix material found in other connective tissues, including some amount of collagen and elastic fibers. This gives strength and flexibility to the tissue. The inorganic matrix consists of mineral salts—mostly calcium salts—that give the tissue hardness. Without adequate organic material in the matrix, the tissue breaks; without adequate inorganic material in the matrix, the tissue bends.
There are three types of cells in bone: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are active in making bone for growth and remodeling. Osteoblasts deposit bone material into the matrix and, after the matrix surrounds them, they continue to live, but in a reduced metabolic state as osteocytes. Osteocytes are found in lacunae of the bone. Osteoclasts are active in breaking down bone for bone remodeling, and they provide access to calcium stored in tissues. Osteoclasts are usually found on the surface of the tissue.
Bone can be divided into two types: compact and spongy. Compact bone is found in the shaft (or diaphysis) of a long bone and the surface of the flat bones, while spongy bone is found in the end (or epiphysis) of a long bone. Compact bone is organized into subunits called osteons, as illustrated in Figure \(9\). A blood vessel and a nerve are found in the center of the structure within the Haversian canal, with radiating circles of lacunae around it known as lamellae. The wavy lines seen between the lacunae are microchannels called canaliculi; they connect the lacunae to aid diffusion between the cells. Spongy bone is made of tiny plates called trabeculae these plates serve as struts to give the spongy bone strength. Over time, these plates can break causing the bone to become less resilient. Bone tissue forms the internal skeleton of vertebrate animals, providing structure to the animal and points of attachment for tendons.
Adipose Tissue
Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a real matrix and only has a few fibers. Adipose tissue is made up of cells called adipocytes that collect and store fat in the form of triglycerides, for energy metabolism. Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic, and they function as cushioning against damage to body organs. Under a microscope, adipose tissue cells appear empty due to the extraction of fat during the processing of the material for viewing, as seen in Figure \(10\). The thin lines in the image are the cell membranes, and the nuclei are the small, black dots at the edges of the cells.
Blood
Blood is considered a connective tissue because it has a matrix, as shown in Figure \(11\). The living cell types are red blood cells (RBC), also called erythrocytes, and white blood cells (WBC), also called leukocytes. The fluid portion of whole blood, its matrix, is commonly called plasma.
The cell found in greatest abundance in blood is the erythrocyte. Erythrocytes are counted in millions in a blood sample: the average number of red blood cells in primates is 4.7 to 5.5 million cells per microliter. Erythrocytes are consistently the same size in a species, but vary in size between species. For example, the average diameter of a primate red blood cell is 7.5 µl, a dog is close at 7.0 µl, but a cat’s RBC diameter is 5.9 µl. Sheep erythrocytes are even smaller at 4.6 µl. Mammalian erythrocytes lose their nuclei and mitochondria when they are released from the bone marrow where they are made. Fish, amphibian, and avian red blood cells maintain their nuclei and mitochondria throughout the cell’s life. The principal job of an erythrocyte is to carry and deliver oxygen to the tissues.
Leukocytes are the predominant white blood cells found in the peripheral blood. Leukocytes are counted in the thousands in the blood with measurements expressed as ranges: primate counts range from 4,800 to 10,800 cells per µl, dogs from 5,600 to 19,200 cells per µl, cats from 8,000 to 25,000 cells per µl, cattle from 4,000 to 12,000 cells per µl, and pigs from 11,000 to 22,000 cells per µl.
Lymphocytes function primarily in the immune response to foreign antigens or material. Different types of lymphocytes make antibodies tailored to the foreign antigens and control the production of those antibodies. Neutrophils are phagocytic cells and they participate in one of the early lines of defense against microbial invaders, aiding in the removal of bacteria that has entered the body. Another leukocyte that is found in the peripheral blood is the monocyte. Monocytes give rise to phagocytic macrophages that clean up dead and damaged cells in the body, whether they are foreign or from the host animal. Two additional leukocytes in the blood are eosinophils and basophils—both help to facilitate the inflammatory response.
The slightly granular material among the cells is a cytoplasmic fragment of a cell in the bone marrow. This is called a platelet or thrombocyte. Platelets participate in the stages leading up to coagulation of the blood to stop bleeding through damaged blood vessels. Blood has a number of functions, but primarily it transports material through the body to bring nutrients to cells and remove waste material from them.
Muscle Tissues
There are three types of muscle in animal bodies: smooth, skeletal, and cardiac. They differ by the presence or absence of striations or bands, the number and location of nuclei, whether they are voluntarily or involuntarily controlled, and their location within the body. The table summarizes these differences.
Table \(3\): Types of Muscles
Type of Muscle Striations Nuclei Control Location
smooth no single, in center involuntary visceral organs
skeletal yes many, at periphery voluntary skeletal muscles
cardiac yes single, in center involuntary heart
Smooth Muscle
Smooth muscle does not have striations in its cells. It has a single, centrally located nucleus, as shown in Figure \(12\). Constriction of smooth muscle occurs under involuntary, autonomic nervous control and in response to local conditions in the tissues. Smooth muscle tissue is also called non-striated as it lacks the banded appearance of skeletal and cardiac muscle. The walls of blood vessels, the tubes of the digestive system, and the tubes of the reproductive systems are composed of mostly smooth muscle.
Skeletal Muscle
Skeletal muscle has striations across its cells caused by the arrangement of the contractile proteins actin and myosin. These muscle cells are relatively long and have multiple nuclei along the edge of the cell. Skeletal muscle is under voluntary, somatic nervous system control and is found in the muscles that move bones. Figure \(12\)illustrates the histology of skeletal muscle.
Cardiac Muscle
Cardiac muscle, shown in Figure \(12\), is found only in the heart. Like skeletal muscle, it has cross striations in its cells, but cardiac muscle has a single, centrally located nucleus. Cardiac muscle is not under voluntary control but can be influenced by the autonomic nervous system to speed up or slow down. An added feature to cardiac muscle cells is a line than extends along the end of the cell as it abuts the next cardiac cell in the row. This line is called an intercalated disc: it assists in passing electrical impulse efficiently from one cell to the next and maintains the strong connection between neighboring cardiac cells.
Nervous Tissues
Nervous tissues are made of cells specialized to receive and transmit electrical impulses from specific areas of the body and to send them to specific locations in the body. The main cell of the nervous system is the neuron, illustrated in Figure \(13\). The large structure with a central nucleus is the cell body of the neuron. Projections from the cell body are either dendrites specialized in receiving input or a single axon specialized in transmitting impulses. Some glial cells are also shown. Astrocytes regulate the chemical environment of the nerve cell, and oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently. Other glial cells that are not shown support the nutritional and waste requirements of the neuron. Some of the glial cells are phagocytic and remove debris or damaged cells from the tissue. A nerve consists of neurons and glial cells.
Link to Learning
Click through the interactive review to learn more about epithelial tissues.
Career Connections: Pathologist
A pathologist is a medical doctor or veterinarian who has specialized in the laboratory detection of disease in animals, including humans. These professionals complete medical school education and follow it with an extensive post-graduate residency at a medical center. A pathologist may oversee clinical laboratories for the evaluation of body tissue and blood samples for the detection of disease or infection. They examine tissue specimens through a microscope to identify cancers and other diseases. Some pathologists perform autopsies to determine the cause of death and the progression of disease.
Summary
The basic building blocks of complex animals are four primary tissues. These are combined to form organs, which have a specific, specialized function within the body, such as the skin or kidney. Organs are organized together to perform common functions in the form of systems. The four primary tissues are epithelia, connective tissues, muscle tissues, and nervous tissues.
Glossary
canaliculus
microchannel that connects the lacunae and aids diffusion between cells
cartilage
type of connective tissue with a large amount of ground substance matrix, cells called chondrocytes, and some amount of fibers
chondrocyte
cell found in cartilage
columnar epithelia
epithelia made of cells taller than they are wide, specialized in absorption
connective tissue
type of tissue made of cells, ground substance matrix, and fibers
cuboidal epithelia
epithelia made of cube-shaped cells, specialized in glandular functions
epithelial tissue
tissue that either lines or covers organs or other tissues
fibrous connective tissue
type of connective tissue with a high concentration of fibers
lacuna
space in cartilage and bone that contains living cells
loose (areolar) connective tissue
type of connective tissue with small amounts of cells, matrix, and fibers; found around blood vessels
matrix
component of connective tissue made of both living and non-living (ground substances) cells
osteon
subunit of compact bone
pseudostratified
layer of epithelia that appears multilayered, but is a simple covering
simple epithelia
single layer of epithelial cells
squamous epithelia
type of epithelia made of flat cells, specialized in aiding diffusion or preventing abrasion
stratified epithelia
multiple layers of epithelial cells
trabecula
tiny plate that makes up spongy bone and gives it strength
transitional epithelia
epithelia that can transition for appearing multilayered to simple; also called uroepithelial | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/41%3A_The_Animal_Body_and_Principles_of_Regulation/41.02%3A_Epithelial_Tissue.txt |
Skills to Develop
• Describe epithelial tissues
• Discuss the different types of connective tissues in animals
• Describe three types of muscle tissues
• Describe nervous tissue
The tissues of multicellular, complex animals are four primary types: epithelial, connective, muscle, and nervous. Recall that tissues are groups of similar cells group of similar cells carrying out related functions. These tissues combine to form organs—like the skin or kidney—that have specific, specialized functions within the body. Organs are organized into organ systems to perform functions; examples include the circulatory system, which consists of the heart and blood vessels, and the digestive system, consisting of several organs, including the stomach, intestines, liver, and pancreas. Organ systems come together to create an entire organism.
Epithelial Tissues
Epithelial tissues cover the outside of organs and structures in the body and line the lumens of organs in a single layer or multiple layers of cells. The types of epithelia are classified by the shapes of cells present and the number of layers of cells. Epithelia composed of a single layer of cells is called simple epithelia; epithelial tissue composed of multiple layers is called stratified epithelia. The table summarizes the different types of epithelial tissues.
Table \(1\): Different Types of Epithelial Tissues
Cell shape Description Location
squamous flat, irregular round shape simple: lung alveoli, capillaries stratified: skin, mouth, vagina
cuboidal cube shaped, central nucleus glands, renal tubules
columnar tall, narrow, nucleus toward base tall, narrow, nucleus along cell simple: digestive tract pseudostratified: respiratory tract
transitional round, simple but appear stratified urinary bladder
Squamous Epithelia
Squamous epithelial cells are generally round, flat, and have a small, centrally located nucleus. The cell outline is slightly irregular, and cells fit together to form a covering or lining. When the cells are arranged in a single layer (simple epithelia), they facilitate diffusion in tissues, such as the areas of gas exchange in the lungs and the exchange of nutrients and waste at blood capillaries.
Figure \(1\) illustrates a layer of squamous cells with their membranes joined together to form an epithelium. Image Figure \(1\) illustrates squamous epithelial cells arranged in stratified layers, where protection is needed on the body from outside abrasion and damage. This is called a stratified squamous epithelium and occurs in the skin and in tissues lining the mouth and vagina.
Cuboidal Epithelia
Cuboidal epithelial cells, shown in Figure \(2\), are cube-shaped with a single, central nucleus. They are most commonly found in a single layer representing a simple epithelia in glandular tissues throughout the body where they prepare and secrete glandular material. They are also found in the walls of tubules and in the ducts of the kidney and liver.
Columnar Epithelia
Columnar epithelial cells are taller than they are wide: they resemble a stack of columns in an epithelial layer, and are most commonly found in a single-layer arrangement. The nuclei of columnar epithelial cells in the digestive tract appear to be lined up at the base of the cells, as illustrated in Figure \(3\). These cells absorb material from the lumen of the digestive tract and prepare it for entry into the body through the circulatory and lymphatic systems.
Columnar epithelial cells lining the respiratory tract appear to be stratified. However, each cell is attached to the base membrane of the tissue and, therefore, they are simple tissues. The nuclei are arranged at different levels in the layer of cells, making it appear as though there is more than one layer, as seen in Figure \(4\). This is called pseudostratified, columnar epithelia. This cellular covering has cilia at the apical, or free, surface of the cells. The cilia enhance the movement of mucous and trapped particles out of the respiratory tract, helping to protect the system from invasive microorganisms and harmful material that has been breathed into the body. Goblet cells are interspersed in some tissues (such as the lining of the trachea). The goblet cells contain mucous that traps irritants, which in the case of the trachea keep these irritants from getting into the lungs.
Transitional Epithelia
Transitional or uroepithelial cells appear only in the urinary system, primarily in the bladder and ureter. These cells are arranged in a stratified layer, but they have the capability of appearing to pile up on top of each other in a relaxed, empty bladder, as illustrated in Figure \(5\). As the urinary bladder fills, the epithelial layer unfolds and expands to hold the volume of urine introduced into it. As the bladder fills, it expands and the lining becomes thinner. In other words, the tissue transitions from thick to thin.
Exercise
Which of the following statements about types of epithelial cells is false?
1. Simple columnar epithelial cells line the tissue of the lung.
2. Simple cuboidal epithelial cells are involved in the filtering of blood in the kidney.
3. Pseudostratisfied columnar epithilia occur in a single layer, but the arrangement of nuclei makes it appear that more than one layer is present.
4. Transitional epithelia change in thickness depending on how full the bladder is.
Answer
A
Connective Tissues
Connective tissues are made up of a matrix consisting of living cells and a non-living substance, called the ground substance. The ground substance is made of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principal cell of connective tissues is the fibroblast. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and, occasionally, leukocytes can be found in some of the tissues. Some tissues have specialized cells that are not found in the others. The matrix in connective tissues gives the tissue its density. When a connective tissue has a high concentration of cells or fibers, it has proportionally a less dense matrix.
The organic portion or protein fibers found in connective tissues are either collagen, elastic, or reticular fibers. Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding tissues. Elastic fibers are made of the protein elastin; this fiber can stretch to one and one half of its length and return to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers are the third type of protein fiber found in connective tissues. This fiber consists of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected. The various types of connective tissues, the types of cells and fibers they are made of, and sample locations of the tissues is summarized in the table.
Table \(2\): Connective Tissues
Tissue Cells Fibers Location
loose/areolar fibroblasts, macrophages, some lymphocytes, some neutrophils few: collagen, elastic, reticular around blood vessels; anchors epithelia
dense, fibrous connective tissue fibroblasts, macrophages, mostly collagen irregular: skin regular: tendons, ligaments
cartilage chondrocytes, chondroblasts hyaline: few collagen fibrocartilage: large amount of collagen shark skeleton, fetal bones, human ears, intervertebral discs
bone osteoblasts, osteocytes, osteoclasts some: collagen, elastic vertebrate skeletons
adipose adipocytes few adipose (fat)
blood red blood cells, white blood cells none blood
Loose/Areolar Connective Tissue
Loose connective tissue, also called areolar connective tissue, has a sampling of all of the components of a connective tissue. As illustrated in Figure \(6\), loose connective tissue has some fibroblasts; macrophages are present as well. Collagen fibers are relatively wide and stain a light pink, while elastic fibers are thin and stain dark blue to black. The space between the formed elements of the tissue is filled with the matrix. The material in the connective tissue gives it a loose consistency similar to a cotton ball that has been pulled apart. Loose connective tissue is found around every blood vessel and helps to keep the vessel in place. The tissue is also found around and between most body organs. In summary, areolar tissue is tough, yet flexible, and comprises membranes.
Fibrous Connective Tissue
Fibrous connective tissues contain large amounts of collagen fibers and few cells or matrix material. The fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly arranged fibrous connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of the skin. Regular fibrous connective tissue, shown in Figure \(7\), is found in tendons (which connect muscles to bones) and ligaments (which connect bones to bones).
Cartilage
Cartilage is a connective tissue with a large amount of the matrix and variable amounts of fibers. The cells, called chondrocytes, make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called lacunae.
A cartilage with few collagen and elastic fibers is hyaline cartilage, illustrated in Figure \(8\). The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine histological stains. Sharks have cartilaginous skeletons, as does nearly the entire human skeleton during a specific pre-birth developmental stage. A remnant of this cartilage persists in the outer portion of the human nose. Hyaline cartilage is also found at the ends of long bones, reducing friction and cushioning the articulations of these bones.
Elastic cartilage has a large amount of elastic fibers, giving it tremendous flexibility. The ears of most vertebrate animals contain this cartilage as do portions of the larynx, or voice box. Fibrocartilage contains a large amount of collagen fibers, giving the tissue tremendous strength. Fibrocartilage comprises the intervertebral discs in vertebrate animals. Hyaline cartilage found in movable joints such as the knee and shoulder becomes damaged as a result of age or trauma. Damaged hyaline cartilage is replaced by fibrocartilage and results in the joints becoming “stiff.”
Bone
Bone, or osseous tissue, is a connective tissue that has a large amount of two different types of matrix material. The organic matrix is similar to the matrix material found in other connective tissues, including some amount of collagen and elastic fibers. This gives strength and flexibility to the tissue. The inorganic matrix consists of mineral salts—mostly calcium salts—that give the tissue hardness. Without adequate organic material in the matrix, the tissue breaks; without adequate inorganic material in the matrix, the tissue bends.
There are three types of cells in bone: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are active in making bone for growth and remodeling. Osteoblasts deposit bone material into the matrix and, after the matrix surrounds them, they continue to live, but in a reduced metabolic state as osteocytes. Osteocytes are found in lacunae of the bone. Osteoclasts are active in breaking down bone for bone remodeling, and they provide access to calcium stored in tissues. Osteoclasts are usually found on the surface of the tissue.
Bone can be divided into two types: compact and spongy. Compact bone is found in the shaft (or diaphysis) of a long bone and the surface of the flat bones, while spongy bone is found in the end (or epiphysis) of a long bone. Compact bone is organized into subunits called osteons, as illustrated in Figure \(9\). A blood vessel and a nerve are found in the center of the structure within the Haversian canal, with radiating circles of lacunae around it known as lamellae. The wavy lines seen between the lacunae are microchannels called canaliculi; they connect the lacunae to aid diffusion between the cells. Spongy bone is made of tiny plates called trabeculae these plates serve as struts to give the spongy bone strength. Over time, these plates can break causing the bone to become less resilient. Bone tissue forms the internal skeleton of vertebrate animals, providing structure to the animal and points of attachment for tendons.
Adipose Tissue
Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a real matrix and only has a few fibers. Adipose tissue is made up of cells called adipocytes that collect and store fat in the form of triglycerides, for energy metabolism. Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic, and they function as cushioning against damage to body organs. Under a microscope, adipose tissue cells appear empty due to the extraction of fat during the processing of the material for viewing, as seen in Figure \(10\). The thin lines in the image are the cell membranes, and the nuclei are the small, black dots at the edges of the cells.
Blood
Blood is considered a connective tissue because it has a matrix, as shown in Figure \(11\). The living cell types are red blood cells (RBC), also called erythrocytes, and white blood cells (WBC), also called leukocytes. The fluid portion of whole blood, its matrix, is commonly called plasma.
The cell found in greatest abundance in blood is the erythrocyte. Erythrocytes are counted in millions in a blood sample: the average number of red blood cells in primates is 4.7 to 5.5 million cells per microliter. Erythrocytes are consistently the same size in a species, but vary in size between species. For example, the average diameter of a primate red blood cell is 7.5 µl, a dog is close at 7.0 µl, but a cat’s RBC diameter is 5.9 µl. Sheep erythrocytes are even smaller at 4.6 µl. Mammalian erythrocytes lose their nuclei and mitochondria when they are released from the bone marrow where they are made. Fish, amphibian, and avian red blood cells maintain their nuclei and mitochondria throughout the cell’s life. The principal job of an erythrocyte is to carry and deliver oxygen to the tissues.
Leukocytes are the predominant white blood cells found in the peripheral blood. Leukocytes are counted in the thousands in the blood with measurements expressed as ranges: primate counts range from 4,800 to 10,800 cells per µl, dogs from 5,600 to 19,200 cells per µl, cats from 8,000 to 25,000 cells per µl, cattle from 4,000 to 12,000 cells per µl, and pigs from 11,000 to 22,000 cells per µl.
Lymphocytes function primarily in the immune response to foreign antigens or material. Different types of lymphocytes make antibodies tailored to the foreign antigens and control the production of those antibodies. Neutrophils are phagocytic cells and they participate in one of the early lines of defense against microbial invaders, aiding in the removal of bacteria that has entered the body. Another leukocyte that is found in the peripheral blood is the monocyte. Monocytes give rise to phagocytic macrophages that clean up dead and damaged cells in the body, whether they are foreign or from the host animal. Two additional leukocytes in the blood are eosinophils and basophils—both help to facilitate the inflammatory response.
The slightly granular material among the cells is a cytoplasmic fragment of a cell in the bone marrow. This is called a platelet or thrombocyte. Platelets participate in the stages leading up to coagulation of the blood to stop bleeding through damaged blood vessels. Blood has a number of functions, but primarily it transports material through the body to bring nutrients to cells and remove waste material from them.
Muscle Tissues
There are three types of muscle in animal bodies: smooth, skeletal, and cardiac. They differ by the presence or absence of striations or bands, the number and location of nuclei, whether they are voluntarily or involuntarily controlled, and their location within the body. The table summarizes these differences.
Table \(3\): Types of Muscles
Type of Muscle Striations Nuclei Control Location
smooth no single, in center involuntary visceral organs
skeletal yes many, at periphery voluntary skeletal muscles
cardiac yes single, in center involuntary heart
Smooth Muscle
Smooth muscle does not have striations in its cells. It has a single, centrally located nucleus, as shown in Figure \(12\). Constriction of smooth muscle occurs under involuntary, autonomic nervous control and in response to local conditions in the tissues. Smooth muscle tissue is also called non-striated as it lacks the banded appearance of skeletal and cardiac muscle. The walls of blood vessels, the tubes of the digestive system, and the tubes of the reproductive systems are composed of mostly smooth muscle.
Skeletal Muscle
Skeletal muscle has striations across its cells caused by the arrangement of the contractile proteins actin and myosin. These muscle cells are relatively long and have multiple nuclei along the edge of the cell. Skeletal muscle is under voluntary, somatic nervous system control and is found in the muscles that move bones. Figure \(12\)illustrates the histology of skeletal muscle.
Cardiac Muscle
Cardiac muscle, shown in Figure \(12\), is found only in the heart. Like skeletal muscle, it has cross striations in its cells, but cardiac muscle has a single, centrally located nucleus. Cardiac muscle is not under voluntary control but can be influenced by the autonomic nervous system to speed up or slow down. An added feature to cardiac muscle cells is a line than extends along the end of the cell as it abuts the next cardiac cell in the row. This line is called an intercalated disc: it assists in passing electrical impulse efficiently from one cell to the next and maintains the strong connection between neighboring cardiac cells.
Nervous Tissues
Nervous tissues are made of cells specialized to receive and transmit electrical impulses from specific areas of the body and to send them to specific locations in the body. The main cell of the nervous system is the neuron, illustrated in Figure \(13\). The large structure with a central nucleus is the cell body of the neuron. Projections from the cell body are either dendrites specialized in receiving input or a single axon specialized in transmitting impulses. Some glial cells are also shown. Astrocytes regulate the chemical environment of the nerve cell, and oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently. Other glial cells that are not shown support the nutritional and waste requirements of the neuron. Some of the glial cells are phagocytic and remove debris or damaged cells from the tissue. A nerve consists of neurons and glial cells.
Link to Learning
Click through the interactive review to learn more about epithelial tissues.
Career Connections: Pathologist
A pathologist is a medical doctor or veterinarian who has specialized in the laboratory detection of disease in animals, including humans. These professionals complete medical school education and follow it with an extensive post-graduate residency at a medical center. A pathologist may oversee clinical laboratories for the evaluation of body tissue and blood samples for the detection of disease or infection. They examine tissue specimens through a microscope to identify cancers and other diseases. Some pathologists perform autopsies to determine the cause of death and the progression of disease.
Summary
The basic building blocks of complex animals are four primary tissues. These are combined to form organs, which have a specific, specialized function within the body, such as the skin or kidney. Organs are organized together to perform common functions in the form of systems. The four primary tissues are epithelia, connective tissues, muscle tissues, and nervous tissues.
Glossary
canaliculus
microchannel that connects the lacunae and aids diffusion between cells
cartilage
type of connective tissue with a large amount of ground substance matrix, cells called chondrocytes, and some amount of fibers
chondrocyte
cell found in cartilage
columnar epithelia
epithelia made of cells taller than they are wide, specialized in absorption
connective tissue
type of tissue made of cells, ground substance matrix, and fibers
cuboidal epithelia
epithelia made of cube-shaped cells, specialized in glandular functions
epithelial tissue
tissue that either lines or covers organs or other tissues
fibrous connective tissue
type of connective tissue with a high concentration of fibers
lacuna
space in cartilage and bone that contains living cells
loose (areolar) connective tissue
type of connective tissue with small amounts of cells, matrix, and fibers; found around blood vessels
matrix
component of connective tissue made of both living and non-living (ground substances) cells
osteon
subunit of compact bone
pseudostratified
layer of epithelia that appears multilayered, but is a simple covering
simple epithelia
single layer of epithelial cells
squamous epithelia
type of epithelia made of flat cells, specialized in aiding diffusion or preventing abrasion
stratified epithelia
multiple layers of epithelial cells
trabecula
tiny plate that makes up spongy bone and gives it strength
transitional epithelia
epithelia that can transition for appearing multilayered to simple; also called uroepithelial
41.03: Connective Tissue
The development of a fertilized egg into a newborn child requires an average of 41 rounds of mitosis ($2^{41} = 2.2 \times 10^{12}$). During this period, the cells produced by mitosis enter different pathways of differentiation; some becoming blood cells, some muscle cells, and so on. There are more than 100 visibly-distinguishable kinds of differentiated cells in the vertebrate animal. These are organized into tissues; the tissues into organs. Groups of organs make up the various systems — digestive, excretory, etc. — of the body (Figure $1$ and Table $1$).
The actual number of differentiated cell types is surely much larger than 100. All lymphocytes, for example, look alike but actually represent a variety of different functional types, e.g., B cells, T cells of various subsets. The neurons of the central nervous system must exist in a thousand or more different functional types, each representing the result of a particular pathway of differentiation. This page will give a brief introduction to the major types of animal tissues.
Table $1$: Classification of Animal Tissues
Epithelial Tissues Linings and Coverings Simple Epithelia
Classifying or Naming Epithelia Stratified Epithelia
Glands Exocrine Glands
Endocrine Glands
Connective Tissues Fluid Connective Tissues Lymph
Blood
Connective Tissues Proper Loose Connective Tissues
Loose Connective Tissues and Inflammation
Dense Connective Tissues
Supportive Connective Tissues Osseous Tissue
Cartilage
Muscle Tissues Non-striated Smooth muscle
Striated Skeletal Muscle
Cardiac Muscle
Nervous Tissues Neurons Multipolar Neurons in CNS
Nerves Nerves of the PNS
Receptors Miessner's and Pacinian Corpuscles
Epithelial
Epithelial tissue is made of closely-packed cells arranged in flat sheets. Epithelia form the surface of the skin, line the various cavities and tubes of the body, and cover the internal organs. Epithelia that form the interface between the internal and external environments. Skin as well as the lining of the mouth and nasal cavity. These are derived from ectoderm. Inner lining of the GI tract, lungs, urinary bladder, exocrine glands, vagina and more. These are derived from endoderm.
The apical surface of these epithelial cells is exposed to the "external environment", the lumen of the organ or the air.
• Mesothelia. These are derived from mesoderm.
• pleura — the outer covering of the lungs and the inner lining of the thoracic (chest) cavity.
• peritoneum — the outer covering of all the abdominal organs and the inner lining of the abdominal cavity.
• pericardium — the outer lining of the heart.
• Endothelia. These are derived from mesoderm. The inner lining of the heart, all blood and lymphatic vessels.
The basolateral surface of all epithelia is exposed to the internal environment - extracellular fluid (ECF). The entire sheet of epithelial cells is attached to a layer of extracellular matrix that is called the basement membrane or, better (because it is not a membrane in the biological sense), the basal lamina.
The function of epithelia always reflects the fact that they are boundaries between masses of cells and a cavity or space. Some examples include:
• The epithelium of the skin protects the underlying tissues from mechanical damage, ultraviolet light, dehydration and invasion by bacteria
• The columnar epithelium of the intestine secretes digestive enzymes into the intestine and absorbs the products of digestion from it.
• An epithelium also lines our air passages and the alveoli of the lungs. It secretes mucus which keeps it from drying out and traps inhaled dust particles. Most of its cells have cilia on their apical surface that propel the mucus with its load of foreign matter back up to the throat.
Muscle
Three kinds of muscle are found in vertebrates. Skeletal muscle is made of long fibers whose contraction provides the force of locomotion and other voluntary body movements. Smooth muscle lines the walls of the hollow structures of the body, such as the intestine, urinary bladder, uterus, and blood vessels. Its contraction, which is involuntary, reduces the size of these hollow organs. The heart is made of cardiac muscle.
Connective
The cells of connective tissue are embedded in a great amount of extracellular material. This matrix is secreted by the cells. It consists of protein fibers embedded in an amorphous mixture of protein-polysaccharide ("proteoglycan") molecules. Supporting connective tissue gives strength, support, and protection to the soft parts of the body.
• cartilage. Example: the outer ear
• bone. The matrix of bone contains collagen fibers and mineral deposits. The most abundant mineral is calcium phosphate, although magnesium, carbonate, and fluoride ions are also present.
Dense connective tissue is often called fibrous connective tissue and include Tendons and Ligaments. Tendons connect muscle to bone with a The matrix is principally Type I collagen, and the fibers are all oriented parallel to each other. Tendons are strong but not elastic. Ligaments attach one bone to another and contain both collagen and also the protein elastin. Elastin permits ligaments to be stretched.
Loose connective tissue is distributed throughout the body. It serves as a packing and binding material for most of our organs. Sheets of loose connective tissue that bind muscles and other structures together are called fascia. Collagen, elastin, and other proteins are found in the matrix of loose connective tissue. Both dense and loose connective tissue are derived from cells called fibroblasts, which secrete the extracellular matrix.
Adipose Tissue
Adipose tissue is "fat". There are two kinds found in mammals: white adipose tissue (WAT) and brown adipose tissue (BAT). The WAT in which the cells, called adipocytes, have become almost filled with oil, which is confined within a single membrane-enclosed droplet. Virtually all of the "fat" in adult humans is white adipose tissue. BAT in which the adipocytes contain many small droplets of oil as well as many mitochondria. White adipose tissue and brown adipose tissue differ in function as well as cellular structure. These differences are described elsehwhere.
New adipocytes in white adipose tissue are formed throughout life from a pool of precursor cells. These are needed to replace those that die (after an average life span of 10 years). Whether the total number of these adipocytes increases in humans becoming fatter as adults is still uncertain. If not, why do so many of us get fatter as we age? Because of the increased size of individual adipocytes as they become filled with oil. The adipocytes of white adipose tissue secrete several hormones, including leptin and adiponectin.
Nerve
Nerve tissue is composed of nerve cells called neurons and glial cells. Neurons are specialized for the conduction of nerve impulses; a typical neuron consists of a cell body which contains the nucleus; a number of short fibers — dendrites — extending from the cell body and a single long fiber, the axon. The nerve impulse is conducted along the axon. The tips of axons meet other neurons at junctions called synapses, muscles (called neuromuscular junctions) and glands.
Glia
Glial cells surround neurons. Once thought to be simply support for neurons (glia = glue), they turn out to serve several important functions. There are three types:
• Schwann cells. These produce the myelin sheath that surrounds many axons in the peripheral nervous system.
• Oligodendrocytes. These produce the myelin sheath that surrounds many axons in the central nervous system (brain and spinal cord).
• Astrocytes. These, often star-shaped cells are clustered around synapses and the nodes of Ranvier where they perform a variety of functions such as:
• modulating the activity of neurons
• supplying neurons with materials (e.g. glucose and lactate) as well as some signaling molecules
• regulating the flow of blood to their region of the brain. It is primarily the metabolic activity of astrocytes that is being measured in brain imaging by positron-emission tomography (PET) and functional magnetic resonance imaging (fMRI).
• pruning away (by phagocytosis) weak synapses
In addition, the central nervous system contains many microglia — mobile cells (macrophages) that respond to damage (e.g., from an infection) by engulfing cell debris and secreting inflammatory cytokines like tumor necrosis factor (TNF-α) and interleukin-1 (IL-1). Microglia are also active in the healthy brain, at least in young mice where, like astrocytes, they engulf synapses thus reducing the number of synapses in the developing brain.
Blood
The bone marrow is the source of all the cells of the blood. These include red blood cells (RBCs or erythrocytes), five kinds of white blood cells (WBCs or leukocytes), and platelets (or thrombocytes). | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/41%3A_The_Animal_Body_and_Principles_of_Regulation/41.03%3A_Connective_Tissue/41.3.01%3A_Animal_Tissues.txt |
Skills to Develop
• Describe epithelial tissues
• Discuss the different types of connective tissues in animals
• Describe three types of muscle tissues
• Describe nervous tissue
The tissues of multicellular, complex animals are four primary types: epithelial, connective, muscle, and nervous. Recall that tissues are groups of similar cells group of similar cells carrying out related functions. These tissues combine to form organs—like the skin or kidney—that have specific, specialized functions within the body. Organs are organized into organ systems to perform functions; examples include the circulatory system, which consists of the heart and blood vessels, and the digestive system, consisting of several organs, including the stomach, intestines, liver, and pancreas. Organ systems come together to create an entire organism.
Epithelial Tissues
Epithelial tissues cover the outside of organs and structures in the body and line the lumens of organs in a single layer or multiple layers of cells. The types of epithelia are classified by the shapes of cells present and the number of layers of cells. Epithelia composed of a single layer of cells is called simple epithelia; epithelial tissue composed of multiple layers is called stratified epithelia. The table summarizes the different types of epithelial tissues.
Table \(1\): Different Types of Epithelial Tissues
Cell shape Description Location
squamous flat, irregular round shape simple: lung alveoli, capillaries stratified: skin, mouth, vagina
cuboidal cube shaped, central nucleus glands, renal tubules
columnar tall, narrow, nucleus toward base tall, narrow, nucleus along cell simple: digestive tract pseudostratified: respiratory tract
transitional round, simple but appear stratified urinary bladder
Squamous Epithelia
Squamous epithelial cells are generally round, flat, and have a small, centrally located nucleus. The cell outline is slightly irregular, and cells fit together to form a covering or lining. When the cells are arranged in a single layer (simple epithelia), they facilitate diffusion in tissues, such as the areas of gas exchange in the lungs and the exchange of nutrients and waste at blood capillaries.
Figure \(1\) illustrates a layer of squamous cells with their membranes joined together to form an epithelium. Image Figure \(1\) illustrates squamous epithelial cells arranged in stratified layers, where protection is needed on the body from outside abrasion and damage. This is called a stratified squamous epithelium and occurs in the skin and in tissues lining the mouth and vagina.
Cuboidal Epithelia
Cuboidal epithelial cells, shown in Figure \(2\), are cube-shaped with a single, central nucleus. They are most commonly found in a single layer representing a simple epithelia in glandular tissues throughout the body where they prepare and secrete glandular material. They are also found in the walls of tubules and in the ducts of the kidney and liver.
Columnar Epithelia
Columnar epithelial cells are taller than they are wide: they resemble a stack of columns in an epithelial layer, and are most commonly found in a single-layer arrangement. The nuclei of columnar epithelial cells in the digestive tract appear to be lined up at the base of the cells, as illustrated in Figure \(3\). These cells absorb material from the lumen of the digestive tract and prepare it for entry into the body through the circulatory and lymphatic systems.
Columnar epithelial cells lining the respiratory tract appear to be stratified. However, each cell is attached to the base membrane of the tissue and, therefore, they are simple tissues. The nuclei are arranged at different levels in the layer of cells, making it appear as though there is more than one layer, as seen in Figure \(4\). This is called pseudostratified, columnar epithelia. This cellular covering has cilia at the apical, or free, surface of the cells. The cilia enhance the movement of mucous and trapped particles out of the respiratory tract, helping to protect the system from invasive microorganisms and harmful material that has been breathed into the body. Goblet cells are interspersed in some tissues (such as the lining of the trachea). The goblet cells contain mucous that traps irritants, which in the case of the trachea keep these irritants from getting into the lungs.
Transitional Epithelia
Transitional or uroepithelial cells appear only in the urinary system, primarily in the bladder and ureter. These cells are arranged in a stratified layer, but they have the capability of appearing to pile up on top of each other in a relaxed, empty bladder, as illustrated in Figure \(5\). As the urinary bladder fills, the epithelial layer unfolds and expands to hold the volume of urine introduced into it. As the bladder fills, it expands and the lining becomes thinner. In other words, the tissue transitions from thick to thin.
Exercise
Which of the following statements about types of epithelial cells is false?
1. Simple columnar epithelial cells line the tissue of the lung.
2. Simple cuboidal epithelial cells are involved in the filtering of blood in the kidney.
3. Pseudostratisfied columnar epithilia occur in a single layer, but the arrangement of nuclei makes it appear that more than one layer is present.
4. Transitional epithelia change in thickness depending on how full the bladder is.
Answer
A
Connective Tissues
Connective tissues are made up of a matrix consisting of living cells and a non-living substance, called the ground substance. The ground substance is made of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principal cell of connective tissues is the fibroblast. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and, occasionally, leukocytes can be found in some of the tissues. Some tissues have specialized cells that are not found in the others. The matrix in connective tissues gives the tissue its density. When a connective tissue has a high concentration of cells or fibers, it has proportionally a less dense matrix.
The organic portion or protein fibers found in connective tissues are either collagen, elastic, or reticular fibers. Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding tissues. Elastic fibers are made of the protein elastin; this fiber can stretch to one and one half of its length and return to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers are the third type of protein fiber found in connective tissues. This fiber consists of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected. The various types of connective tissues, the types of cells and fibers they are made of, and sample locations of the tissues is summarized in the table.
Table \(2\): Connective Tissues
Tissue Cells Fibers Location
loose/areolar fibroblasts, macrophages, some lymphocytes, some neutrophils few: collagen, elastic, reticular around blood vessels; anchors epithelia
dense, fibrous connective tissue fibroblasts, macrophages, mostly collagen irregular: skin regular: tendons, ligaments
cartilage chondrocytes, chondroblasts hyaline: few collagen fibrocartilage: large amount of collagen shark skeleton, fetal bones, human ears, intervertebral discs
bone osteoblasts, osteocytes, osteoclasts some: collagen, elastic vertebrate skeletons
adipose adipocytes few adipose (fat)
blood red blood cells, white blood cells none blood
Loose/Areolar Connective Tissue
Loose connective tissue, also called areolar connective tissue, has a sampling of all of the components of a connective tissue. As illustrated in Figure \(6\), loose connective tissue has some fibroblasts; macrophages are present as well. Collagen fibers are relatively wide and stain a light pink, while elastic fibers are thin and stain dark blue to black. The space between the formed elements of the tissue is filled with the matrix. The material in the connective tissue gives it a loose consistency similar to a cotton ball that has been pulled apart. Loose connective tissue is found around every blood vessel and helps to keep the vessel in place. The tissue is also found around and between most body organs. In summary, areolar tissue is tough, yet flexible, and comprises membranes.
Fibrous Connective Tissue
Fibrous connective tissues contain large amounts of collagen fibers and few cells or matrix material. The fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly arranged fibrous connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of the skin. Regular fibrous connective tissue, shown in Figure \(7\), is found in tendons (which connect muscles to bones) and ligaments (which connect bones to bones).
Cartilage
Cartilage is a connective tissue with a large amount of the matrix and variable amounts of fibers. The cells, called chondrocytes, make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called lacunae.
A cartilage with few collagen and elastic fibers is hyaline cartilage, illustrated in Figure \(8\). The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine histological stains. Sharks have cartilaginous skeletons, as does nearly the entire human skeleton during a specific pre-birth developmental stage. A remnant of this cartilage persists in the outer portion of the human nose. Hyaline cartilage is also found at the ends of long bones, reducing friction and cushioning the articulations of these bones.
Elastic cartilage has a large amount of elastic fibers, giving it tremendous flexibility. The ears of most vertebrate animals contain this cartilage as do portions of the larynx, or voice box. Fibrocartilage contains a large amount of collagen fibers, giving the tissue tremendous strength. Fibrocartilage comprises the intervertebral discs in vertebrate animals. Hyaline cartilage found in movable joints such as the knee and shoulder becomes damaged as a result of age or trauma. Damaged hyaline cartilage is replaced by fibrocartilage and results in the joints becoming “stiff.”
Bone
Bone, or osseous tissue, is a connective tissue that has a large amount of two different types of matrix material. The organic matrix is similar to the matrix material found in other connective tissues, including some amount of collagen and elastic fibers. This gives strength and flexibility to the tissue. The inorganic matrix consists of mineral salts—mostly calcium salts—that give the tissue hardness. Without adequate organic material in the matrix, the tissue breaks; without adequate inorganic material in the matrix, the tissue bends.
There are three types of cells in bone: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are active in making bone for growth and remodeling. Osteoblasts deposit bone material into the matrix and, after the matrix surrounds them, they continue to live, but in a reduced metabolic state as osteocytes. Osteocytes are found in lacunae of the bone. Osteoclasts are active in breaking down bone for bone remodeling, and they provide access to calcium stored in tissues. Osteoclasts are usually found on the surface of the tissue.
Bone can be divided into two types: compact and spongy. Compact bone is found in the shaft (or diaphysis) of a long bone and the surface of the flat bones, while spongy bone is found in the end (or epiphysis) of a long bone. Compact bone is organized into subunits called osteons, as illustrated in Figure \(9\). A blood vessel and a nerve are found in the center of the structure within the Haversian canal, with radiating circles of lacunae around it known as lamellae. The wavy lines seen between the lacunae are microchannels called canaliculi; they connect the lacunae to aid diffusion between the cells. Spongy bone is made of tiny plates called trabeculae these plates serve as struts to give the spongy bone strength. Over time, these plates can break causing the bone to become less resilient. Bone tissue forms the internal skeleton of vertebrate animals, providing structure to the animal and points of attachment for tendons.
Adipose Tissue
Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a real matrix and only has a few fibers. Adipose tissue is made up of cells called adipocytes that collect and store fat in the form of triglycerides, for energy metabolism. Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic, and they function as cushioning against damage to body organs. Under a microscope, adipose tissue cells appear empty due to the extraction of fat during the processing of the material for viewing, as seen in Figure \(10\). The thin lines in the image are the cell membranes, and the nuclei are the small, black dots at the edges of the cells.
Blood
Blood is considered a connective tissue because it has a matrix, as shown in Figure \(11\). The living cell types are red blood cells (RBC), also called erythrocytes, and white blood cells (WBC), also called leukocytes. The fluid portion of whole blood, its matrix, is commonly called plasma.
The cell found in greatest abundance in blood is the erythrocyte. Erythrocytes are counted in millions in a blood sample: the average number of red blood cells in primates is 4.7 to 5.5 million cells per microliter. Erythrocytes are consistently the same size in a species, but vary in size between species. For example, the average diameter of a primate red blood cell is 7.5 µl, a dog is close at 7.0 µl, but a cat’s RBC diameter is 5.9 µl. Sheep erythrocytes are even smaller at 4.6 µl. Mammalian erythrocytes lose their nuclei and mitochondria when they are released from the bone marrow where they are made. Fish, amphibian, and avian red blood cells maintain their nuclei and mitochondria throughout the cell’s life. The principal job of an erythrocyte is to carry and deliver oxygen to the tissues.
Leukocytes are the predominant white blood cells found in the peripheral blood. Leukocytes are counted in the thousands in the blood with measurements expressed as ranges: primate counts range from 4,800 to 10,800 cells per µl, dogs from 5,600 to 19,200 cells per µl, cats from 8,000 to 25,000 cells per µl, cattle from 4,000 to 12,000 cells per µl, and pigs from 11,000 to 22,000 cells per µl.
Lymphocytes function primarily in the immune response to foreign antigens or material. Different types of lymphocytes make antibodies tailored to the foreign antigens and control the production of those antibodies. Neutrophils are phagocytic cells and they participate in one of the early lines of defense against microbial invaders, aiding in the removal of bacteria that has entered the body. Another leukocyte that is found in the peripheral blood is the monocyte. Monocytes give rise to phagocytic macrophages that clean up dead and damaged cells in the body, whether they are foreign or from the host animal. Two additional leukocytes in the blood are eosinophils and basophils—both help to facilitate the inflammatory response.
The slightly granular material among the cells is a cytoplasmic fragment of a cell in the bone marrow. This is called a platelet or thrombocyte. Platelets participate in the stages leading up to coagulation of the blood to stop bleeding through damaged blood vessels. Blood has a number of functions, but primarily it transports material through the body to bring nutrients to cells and remove waste material from them.
Muscle Tissues
There are three types of muscle in animal bodies: smooth, skeletal, and cardiac. They differ by the presence or absence of striations or bands, the number and location of nuclei, whether they are voluntarily or involuntarily controlled, and their location within the body. The table summarizes these differences.
Table \(3\): Types of Muscles
Type of Muscle Striations Nuclei Control Location
smooth no single, in center involuntary visceral organs
skeletal yes many, at periphery voluntary skeletal muscles
cardiac yes single, in center involuntary heart
Smooth Muscle
Smooth muscle does not have striations in its cells. It has a single, centrally located nucleus, as shown in Figure \(12\). Constriction of smooth muscle occurs under involuntary, autonomic nervous control and in response to local conditions in the tissues. Smooth muscle tissue is also called non-striated as it lacks the banded appearance of skeletal and cardiac muscle. The walls of blood vessels, the tubes of the digestive system, and the tubes of the reproductive systems are composed of mostly smooth muscle.
Skeletal Muscle
Skeletal muscle has striations across its cells caused by the arrangement of the contractile proteins actin and myosin. These muscle cells are relatively long and have multiple nuclei along the edge of the cell. Skeletal muscle is under voluntary, somatic nervous system control and is found in the muscles that move bones. Figure \(12\)illustrates the histology of skeletal muscle.
Cardiac Muscle
Cardiac muscle, shown in Figure \(12\), is found only in the heart. Like skeletal muscle, it has cross striations in its cells, but cardiac muscle has a single, centrally located nucleus. Cardiac muscle is not under voluntary control but can be influenced by the autonomic nervous system to speed up or slow down. An added feature to cardiac muscle cells is a line than extends along the end of the cell as it abuts the next cardiac cell in the row. This line is called an intercalated disc: it assists in passing electrical impulse efficiently from one cell to the next and maintains the strong connection between neighboring cardiac cells.
Nervous Tissues
Nervous tissues are made of cells specialized to receive and transmit electrical impulses from specific areas of the body and to send them to specific locations in the body. The main cell of the nervous system is the neuron, illustrated in Figure \(13\). The large structure with a central nucleus is the cell body of the neuron. Projections from the cell body are either dendrites specialized in receiving input or a single axon specialized in transmitting impulses. Some glial cells are also shown. Astrocytes regulate the chemical environment of the nerve cell, and oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently. Other glial cells that are not shown support the nutritional and waste requirements of the neuron. Some of the glial cells are phagocytic and remove debris or damaged cells from the tissue. A nerve consists of neurons and glial cells.
Link to Learning
Click through the interactive review to learn more about epithelial tissues.
Career Connections: Pathologist
A pathologist is a medical doctor or veterinarian who has specialized in the laboratory detection of disease in animals, including humans. These professionals complete medical school education and follow it with an extensive post-graduate residency at a medical center. A pathologist may oversee clinical laboratories for the evaluation of body tissue and blood samples for the detection of disease or infection. They examine tissue specimens through a microscope to identify cancers and other diseases. Some pathologists perform autopsies to determine the cause of death and the progression of disease.
Summary
The basic building blocks of complex animals are four primary tissues. These are combined to form organs, which have a specific, specialized function within the body, such as the skin or kidney. Organs are organized together to perform common functions in the form of systems. The four primary tissues are epithelia, connective tissues, muscle tissues, and nervous tissues.
Glossary
canaliculus
microchannel that connects the lacunae and aids diffusion between cells
cartilage
type of connective tissue with a large amount of ground substance matrix, cells called chondrocytes, and some amount of fibers
chondrocyte
cell found in cartilage
columnar epithelia
epithelia made of cells taller than they are wide, specialized in absorption
connective tissue
type of tissue made of cells, ground substance matrix, and fibers
cuboidal epithelia
epithelia made of cube-shaped cells, specialized in glandular functions
epithelial tissue
tissue that either lines or covers organs or other tissues
fibrous connective tissue
type of connective tissue with a high concentration of fibers
lacuna
space in cartilage and bone that contains living cells
loose (areolar) connective tissue
type of connective tissue with small amounts of cells, matrix, and fibers; found around blood vessels
matrix
component of connective tissue made of both living and non-living (ground substances) cells
osteon
subunit of compact bone
pseudostratified
layer of epithelia that appears multilayered, but is a simple covering
simple epithelia
single layer of epithelial cells
squamous epithelia
type of epithelia made of flat cells, specialized in aiding diffusion or preventing abrasion
stratified epithelia
multiple layers of epithelial cells
trabecula
tiny plate that makes up spongy bone and gives it strength
transitional epithelia
epithelia that can transition for appearing multilayered to simple; also called uroepithelial
41.04: Muscle Tissue
Animals use muscles to convert the chemical energy of ATP into mechanical work. Three different kinds of muscles are found in vertebrate animals.
• Heart muscle also called cardiac muscle makes up the wall of the heart. Throughout our life, it contracts some 70 times per minute pumping about 5 liters of blood each minute.
• Smooth muscle is found in the walls of all the hollow organs of the body (except the heart). Its contraction reduces the size of these structures. Thus it
• regulates the flow of blood in the arteries
• moves your breakfast along through your gastrointestinal tract
• expels urine from your urinary bladder
• sends babies out into the world from the uterus
• regulates the flow of air through the lungs
The contraction of smooth muscle is generally not under voluntary control.
• Skeletal muscle, as its name implies, is the muscle attached to the skeleton. It is also called striated muscle. The contraction of skeletal muscle is under voluntary control.
Anatomy of Skeletal Muscle
A single skeletal muscle, such as the triceps muscle, is attached at its
• origin to a large area of bone; in this case, the humerus.
• at its other end, the insertion, it tapers into a glistening white tendon which, in this case, is attached to the ulna, one of the bones of the lower arm.
As the triceps contracts, the insertion is pulled toward the origin and the arm is straightened or extended at the elbow. Thus the triceps is an extensor. Because skeletal muscle exerts force only when it contracts, a second muscle - a flexor - is needed to flex or bend the joint. The biceps muscle is the flexor of the lower arm. Together, the biceps and triceps make up an antagonistic pair of muscles. Similar pairs, working antagonistically across other joints, provide for almost all the movement of the skeleton.
The Muscle Fiber
Skeletal muscle is made up of thousands of cylindrical muscle fibers often running all the way from origin to insertion. The fibers are bound together by connective tissue through which run blood vessels and nerves.Each muscle fibers contains:
• an array of myofibrils that are stacked lengthwise and run the entire length of the fiber;
• mitochondria;
• an extensive smooth endoplasmic reticulum (SER);
• many nuclei (thus each skeletal muscle fiber is a syncytium).
The multiple nuclei arise from the fact that each muscle fiber develops from the fusion of many cells (called myoblasts). The number of fibers is probably fixed early in life. This is regulated by myostatin, a cytokine that is synthesized in muscle cells (and circulates as a hormone later in life). Myostatin suppresses skeletal muscle development. (Cytokines secreted by a cell type that inhibit proliferation of that same type of cell are called chalones.) Cattle and mice with inactivating mutations in their myostatin genes develop much larger muscles. Some athletes and other remarkably strong people have been found to carry one mutant myostatin gene. These discoveries have already led to the growth of an illicit market in drugs supposedly able to suppress myostatin.
In adults, increased muscle mass comes about through an increase in the thickness of the individual fibers and increase in the amount of connective tissue. In the mouse, at least, fibers increase in size by attracting more myoblasts to fuse with them. The fibers attract more myoblasts by releasing the cytokine interleukin 4 (IL-4). Anything that lowers the level of myostatin also leads to an increase in fiber size.
Because a muscle fiber is not a single cell, its parts are often given special names such as
• sarcolemma for plasma membrane
• sarcoplasmic reticulum for endoplasmic reticulum
• sarcosomes for mitochondria
• sarcoplasm for cytoplasm
Although this tends to obscure the essential similarity in structure and function of these structures and those found in other cells.
The nuclei and mitochondria are located just beneath the plasma membrane. The endoplasmic reticulum extends between the myofibrils. Seen from the side under the microscope, skeletal muscle fibers show a pattern of cross banding, which gives rise to the other name: striated muscle. The striated appearance of the muscle fiber is created by a pattern of alternating dark A bands and light I bands.
• The A bands are bisected by the H zone running through the center of which is the M line.
• The I bands are bisected by the Z disk.
Each myofibril is made up of arrays of parallel filaments.
• The thick filaments have a diameter of about 15 nm. They are composed of the protein myosin.
• The thin filaments have a diameter of about 5 nm. They are composed chiefly of the protein actin along with smaller amounts of two other proteins - troponin and tropomyosin.
The anatomy of a sarcomere
The entire array of thick and thin filaments between the Z disks is called a sarcomere.
• The thick filaments produce the dark A band.
• The thin filaments extend in each direction from the Z disk. Where they do not overlap the thick filaments, they create the light I band.
• The H zone is that portion of the A band where the thick and thin filaments do not overlap.
• The M line runs through the exact center of the sarcomere. Molecules of the giant protein, titin, extend from the M line to the Z disk. One of its functions is to provide elasticity to the muscle. It also provides a scaffold for the assembly of a precise number of myosin molecules in the thick filament (294 in one case). It may also dictate the number of actin molecules in the thin filaments.
Shortening of the sarcomeres in a myofibril produces the shortening of the myofibril and, in turn, of the muscle fiber of which it is a part. [This electron micrograph of a single sarcomere was kindly provided by Dr. H. E. Huxley.]
Activation of Skeletal Muscle
The contraction of skeletal muscle is controlled by the nervous system. The Dying Lioness (an Assyrian relief dating from about 650 B.C.) shows this vividly. Injury to the spinal cord has paralyzed the otherwise undamaged hind legs. In this respect, skeletal muscle differs from smooth and cardiac muscle. Both cardiac and smooth muscle can contract without being stimulated by the nervous system. Nerves of the autonomic branch of the nervous system lead to both smooth and cardiac muscle, but their effect is one of moderating the rate and/or strength of contraction.
The Neuromuscular Junction
Nerve impulses (action potentials) traveling down the motor neurons of the sensory-somatic branch of the nervous system cause the skeletal muscle fibers at which they terminate to contract. The junction between the terminal of a motor neuron and a muscle fiber is called the neuromuscular junction. It is simply one kind of synapse. (The neuromuscular junction is also called the myoneural junction.)
The terminals of motor axons contain thousands of vesicles filled with acetylcholine (ACh). Many of these can be seen in the electron micrograph on the left (courtesy of Prof. B. Katz).
When an action potential reaches the axon terminal, hundreds of these vesicles discharge their ACh onto a specialized area of postsynaptic membrane on the muscle fiber (the folded membrane running diagonally upward from the lower left). This area contains a cluster of transmembrane channels that are opened by ACh and let sodium ions (Na+) diffuse in.
The interior of a resting muscle fiber has a resting potential of about −95 mV. The influx of sodium ions reduces the charge, creating an end plate potential. If the end plate potential reaches the threshold voltage (approximately −50 mV), sodium ions flow in with a rush and an action potential is created in the fiber. The action potential sweeps down the length of the fiber just as it does in an axon. No visible change occurs in the muscle fiber during (and immediately following) the action potential. This period, called the latent period, lasts from 3–10 msec.
Before the latent period is over,
• the enzyme acetylcholinesterase
• breaks down the ACh in the neuromuscular junction (at a speed of 25,000 molecules per second)
• the sodium channels close, and
• the field is cleared for the arrival of another nerve impulse.
• the resting potential of the fiber is restored by an outflow of potassium ions.
The brief (1–2 msec) period needed to restore the resting potential is called the refractory period.
Tetanus
The process of contracting takes some 50 msec; relaxation of the fiber takes another 50–100 msec. Because the refractory period is so much shorter than the time needed for contraction and relaxation, the fiber can be maintained in the contracted state so long as it is stimulated frequently enough (e.g., 50 stimuli per second). Such sustained contraction is called tetanus.
In the above figure:
• When shocks are given at 1/sec, the muscle responds with a single twitch.
• At 5/sec and 10/sec, the individual twitches begin to fuse together, a phenomenon called clonus.
• At 50 shocks per second, the muscle goes into the smooth, sustained contraction of tetanus.
Clonus and tetanus are possible because the refractory period is much briefer than the time needed to complete a cycle of contraction and relaxation. Note that the amount of contraction is greater in clonus and tetanus than in a single twitch.
As we normally use our muscles, the individual fibers go into tetanus for brief periods rather than simply undergoing single twitches.
The Sliding-Filament Model
Each molecule of myosin in the thick filaments contains a globular subunit called the myosin head. The myosin heads have binding sites for the actin molecules in the thin filaments and ATP. Activation of the muscle fiber causes the myosin heads to bind to actin. An allosteric change occurs which draws the thin filament a short distance (~10 nm) past the thick filament. Then the linkages break (for which ATP is needed) and reform farther along the thin filament to repeat the process. As a result, the filaments are pulled past each other in a ratchetlike action. There is no shortening, thickening, or folding of the individual filaments.
Electron microscopy supports this model.
As a muscle contracts,
• the Z disks come closer together
• the width of the I bands decreases
• the width of the H zones decreases
• there is no change in the width of the A band
Conversely, as a muscle is stretched,
• the width of the I bands and H zones increases
• but there is still no change in the width of the A band
Coupling Excitation to Contraction
Calcium ions (Ca2+) link action potentials in a muscle fiber to contraction.
• In resting muscle fibers, Ca2+ is stored in the endoplasmic (sarcoplasmic) reticulum.
• Spaced along the plasma membrane (sarcolemma) of the muscle fiber are inpocketings of the membrane that form "T-tubules". These tubules plunge repeatedly into the interior of the fiber.
• The T-tubules terminate near the calcium-filled sacs of the sarcoplasmic reticulum.
• Each action potential created at the neuromuscular junction sweeps quickly along the sarcolemma and is carried into the T-tubules.
• The arrival of the action potential at the ends of the T-tubules triggers the release of Ca2+.
• The Ca2+ diffuses among the thick and thin filaments where it
• binds to troponin on the thin filaments.
• This turns on the interaction between actin and myosin and the sarcomere contracts.
• Because of the speed of the action potential (milliseconds), the action potential arrives virtually simultaneously at the ends of all the T-tubules, ensuring that all sarcomeres contract in unison.
• When the process is over, the calcium is pumped back into the sarcoplasmic reticulum using a Ca2+ ATPase.
Isotonic versus Isometric Contractions
If a stimulated muscle is held so that it cannot shorten, it simply exerts tension. This is called an isometric ("same length") contraction. If the muscle is allowed to shorten, the contraction is called isotonic ("same tension").
Motor Units
All motor neurons leading to skeletal muscles have branching axons, each of which terminates in a neuromuscular junction with a single muscle fiber. Nerve impulses passing down a single motor neuron will thus trigger contraction in all the muscle fibers at which the branches of that neuron terminate. This minimum unit of contraction is called the motor unit.
The size of the motor unit is small in muscles over which we have precise control. Examples:
• a single motor neuron triggers fewer than 10 fibers in the muscles controlling eye movements
• the motor units of the muscles controlling the larynx are as small as 2–3 fibers per motor neuron
• In contrast, a single motor unit for a muscle like the gastrocnemius (calf) muscle may include 1000–2000 fibers (scattered uniformly through the muscle).
Although the response of a motor unit is all-or-none, the strength of the response of the entire muscle is determined by the number of motor units activated. Even at rest, most of our skeletal muscles are in a state of partial contraction called tonus. Tonus is maintained by the activation of a few motor units at all times even in resting muscle. As one set of motor units relaxes, another set takes over.
Fueling Muscle Contraction
ATP is the immediate source of energy for muscle contraction. Although a muscle fiber contains only enough ATP to power a few twitches, its ATP "pool" is replenished as needed. There are three sources of high-energy phosphate to keep the ATP pool filled.
• creatine phosphate
• glycogen
• cellular respiration in the mitochondria of the fibers.
Creatine phosphate
The phosphate group in creatine phosphate is attached by a "high-energy" bond like that in ATP. Creatine phosphate derives its high-energy phosphate from ATP and can donate it back to ADP to form ATP.
Creatine phosphate + ADP creatine + ATP
The pool of creatine phosphate in the fiber is about 10 times larger than that of ATP and thus serves as a modest reservoir of ATP.
Glycogen
Skeletal muscle fibers contain about 1% glycogen. The muscle fiber can degrade this glycogen by glycogenolysis producing glucose-1-phosphate. This enters the glycolytic pathway to yield two molecules of ATP for each pair of lactic acid molecules produced. Not much, but enough to keep the muscle functioning if it fails to receive sufficient oxygen to meet its ATP needs by respiration.
However, this source is limited and eventually the muscle must depend on cellular respiration.
Cellular respiration
Cellular respiration not only is required to meet the ATP needs of a muscle engaged in prolonged activity (thus causing more rapid and deeper breathing), but is also required afterwards to enable the body to resynthesize glycogen from the lactic acid produced earlier (deep breathing continues for a time after exercise is stopped). The body must repay its oxygen debt.
Type I vs. Type II Fibers
Several different types of muscle fiber can be found in most skeletal muscles: Type I and and 3 subtypes of Type II fibers. Each type differs in the myosin it uses and also in its structure and biochemistry.
Type I Fibers
• loaded with mitochondria
• depend on cellular respiration for ATP production
• fatty acids the major energy source
• resistant to fatigue
• rich in myoglobin and hence red in color (the "dark" meat of the turkey)
• activated by small-diameter, thus slow-conducting, motor neurons
• also known as "slow-twitch" fibers
• dominant in muscles used in activities requiring endurance (leg muscles) and those that depend on tonus, e.g., those responsible for posture
Type IIb Fibers
• few mitochondria
• rich in glycogen
• depend on creatine phosphate and glycolysis for ATP production
• fatigue easily with the production of lactic acid
• low in myoglobin hence whitish in color (the white meat of the turkey)
• activated by large-diameter, thus fast-conducting, motor neurons
• also known as "fast-twitch" fibers
• dominant in muscles used for rapid movement, e.g., those moving the eyeballs.
The other subtypes of Type II fibers have properties intermediate between those of Type IIb and Type I.
Most skeletal muscles contain some mixture of Type I and Type II fibers, but a single motor unit always contains one type or the other, never both.
In mice, the number of Type I vs Type II fibers can be changed with exercise and drug treatment. Whether the same holds true for humans remains to be seen. Perhaps training in humans does not alter the number of fibers of a particular type but may increase the diameter of one type (e.g., Type I in marathoners, Type IIb in weight lifters) at the expense of the other types.
Cardiac Muscle
Cardiac or heart muscle resembles skeletal muscle in some ways: it is striated and each cell contains sarcomeres with sliding filaments of actin and myosin. However, cardiac muscle has a number of unique features that reflect its function of pumping blood.
• The myofibrils of each cell (and cardiac muscle is made of single cells — each with a single nucleus) are branched.
• The branches interlock with those of adjacent fibers by adherens junctions. These strong junctions enable the heart to contract forcefully without ripping the fibers apart.
This electron micrograph (reproduced with permission from Keith R. Porter and Mary A. Bonneville, An Introduction to the Fine Structure of Cells and Tissues, 4th ed., Lea & Febiger, Philadelphia, 1973) shows an adherens junction and several of the other features listed here.
• The action potential that triggers the heartbeat is generated within the heart itself. Motor nerves (of the autonomic nervous system) do run to the heart, but their effect is simply to modulate — increase or decrease — the intrinsic rate and the strength of the heartbeat. Even if the nerves are destroyed (as they are in a transplanted heart), the heart continues to beat.
• The action potential that drives contraction of the heart passes from fiber to fiber through gap junctions.
• Significance: all the fibers contract in a synchronous wave that sweeps from the atria down through the ventricles and pumps blood out of the heart. Anything that interferes with this synchronous wave (such as damage to part of the heart muscle from a heart attack) may cause the fibers of the heart to beat at random — called fibrillation. Fibrillation is the ultimate cause of most deaths, and its reversal is the function of defibrillators that are part of the equipment in ambulances, hospital emergency rooms, and even on U.S. air lines.
• The refractory period in heart muscle is longer than the period it takes for the muscle to contract (systole) and relax (diastole). Thus tetanus is not possible (a good thing, too!).
• Cardiac muscle has a much richer supply of mitochondria than skeletal muscle. This reflects its greater dependence on cellular respiration for ATP.
• Cardiac muscle has little glycogen and gets little benefit from glycolysis when the supply of oxygen is limited.
• Thus anything that interrupts the flow of oxygenated blood to the heart leads quickly to damage - even death - of the affected part. This is what happens in heart attacks.
Smooth Muscle
Smooth muscle is made of single, spindle-shaped cells. It gets its name because no striations are visible in them. Nonetheless, each smooth muscle cell contains thick (myosin) and thin (actin) filaments that slide against each other to produce contraction of the cell. The thick and thin filaments are anchored near the plasma membrane (with the help of intermediate filaments).
Smooth muscle (like cardiac muscle) does not depend on motor neurons to be stimulated. However, motor neurons (of the autonomic system) reach smooth muscle and can stimulate it or relax it depending on the neurotransmitter they release (e.g. noradrenaline or nitric oxide, NO).
Smooth muscle can also be made to contract
• by other substances released in the vicinity (paracrine stimulation)
• Example: release of histamine causes contraction of the smooth muscle lining our air passages (triggering an attack of asthma)
• by hormones circulating in the blood
• Example: oxytocin reaching the uterus stimulates it to contract to begin childbirth.
The contraction of smooth muscle tends to be slower than that of striated muscle. It also is often sustained for long periods. This, too, is called tonus but the mechanism is not like that in skeletal muscle.
Muscle Diseases
The Muscular Dystrophies (MD)
Together myosin, actin, tropomyosin, and troponin make up over three-quarters of the protein in muscle fibers. Some two dozen other proteins make up the rest. These serve such functions as attaching and organizing the filaments in the sarcomere and connecting the sarcomeres to the plasma membrane and the extracellular matrix. Mutations in the genes encoding these proteins may produce defective proteins and resulting defects in the muscles.
Among the most common of the muscular dystrophies are those caused by mutations in the gene for dystrophin. The gene for dystrophin is huge, containing 79 exons spread out over 2.4 million base pairs of DNA. Thus this single gene represents about 0.1% of the entire human genome (3 x 109 bp) and is almost half the size of the entire genome of E. coli!
• Duchenne muscular dystrophy (DMD)
Deletions or nonsense mutations that cause a frameshift usually introduce premature termination codons (PTCs) in the resulting mRNA. Thus at best only a fragment of dystrophin is synthesized and DMD, a very severe form of the disease, results.
• Becker muscular dystrophy (BMD).
If the deletion simply removes certain exons but preserves the correct reading frame, a slightly-shortened protein results that produces BMD, a milder form of the disease.
The gene for dystrophin is on the X chromosome, so these two diseases strike males in a typical X-linked pattern of inheritance.
A treatment for DMD
Deletions of one or more exons in the huge dystrophin gene are the cause of most of the cases of DMD. Exon 50 is a particularly notorious offender. When it is deleted, splicing of the pre-mRNA introduces a frameshift which then introduces a premature termination codon resulting in no functional dystrophin synthesized ("B"). However, an antisense oligonucleotide targeted to exon 51 causes the splicing mechanism to skip over it resulting in the stitching together of exons 49 and 52. This restores the correct reading frame so that only a slightly-altered version of dystrophin is produced, i.e., a BMD-type dystrophin ("C"). Seventeen weeks of weekly injections of 12 young DMD patients in the Netherlands with the oligonucleotide caused their muscles to synthesize sufficient amounts of dystrophin to enable 8 of them to walk better than before. (See Goemans, N., et al., in the 21 April 2011 issue of The New England Journal of Medicine.
Three research groups have used the CRISPR-Cas9 genome editing system to remove a mutated exon in DMD mice. The treatment restored dystrophin synthesis and improved skeletal and cardiac muscle function in the mice.
Myasthenia Gravis
Myasthenia gravis is an autoimmune disorder affecting the neuromuscular junction. Patients have smaller end plate potentials (EPPs) than normal. With repeated stimulation, the EPPs become too small to trigger further action potentials and the fiber ceases to contract. Administration of an inhibitor of acetylcholinesterase temporarily can restore contractility by allowing more ACh to remain at the site.
Patients with myasthenia gravis have only 20% or so of the number of ACh receptors found in normal neuromuscular junctions. This loss appears to be caused by antibodies directed against the receptors. Some evidence:
• A disease resembling myasthenia gravis can be induced in experimental animals by immunizing them with purified ACh receptors.
• Anti-ACh receptor antibodies are found in the serum of human patients.
• Experimental animals injected with serum from human patients develop the signs of myasthenia gravis.
• Newborns of mothers with myasthenia gravis often show mild signs of the disease for a short time after their birth. This is the result of the transfer of the mother's antibodies across the placenta during gestation.
The reason some people develop autoimmune antibodies against the ACh receptor is unknown.
The Cardiac Myopathies
Cardiac muscle, like skeletal muscle, contains many proteins in addition to actin and myosin. Mutations in the genes for these may cause the wall of the heart to become weakened and, in due course, enlarged. Among the genes that have been implicated in these diseases are those encoding:
• actin
• two types of myosin
• troponin
• tropomyosin
• myosin-binding protein C (which links myosin to titin)
The severity of the disease varies with the particular mutation causing it (over 100 have been identified so far) . Some mutations are sufficiently dangerous that they can lead to sudden catastrophic heart failure in seemingly healthy and active young adults. | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/41%3A_The_Animal_Body_and_Principles_of_Regulation/41.04%3A_Muscle_Tissue/41.4B%3A_Muscles.txt |
Skills to Develop
• List and describe the functions of the structural components of a neuron
• List and describe the four main types of neurons
• Compare the functions of different types of glial cells
Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in Figure \(1\). Some organisms, like sea sponges, lack a true nervous system. Others, like jellyfish, lack a true brain and instead have a system of separate but connected nerve cells (neurons) called a “nerve net.” Echinoderms such as sea stars have nerve cells that are bundled into fibers called nerves. Flatworms of the phylum Platyhelminthes have both a central nervous system (CNS), made up of a small “brain” and two nerve cords, and a peripheral nervous system (PNS) containing a system of nerves that extend throughout the body. The insect nervous system is more complex but also fairly decentralized. It contains a brain, ventral nerve cord, and ganglia (clusters of connected neurons). These ganglia can control movements and behaviors without input from the brain. Octopi may have the most complicated of invertebrate nervous systems—they have neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species.
Compared to invertebrates, vertebrate nervous systems are more complex, centralized, and specialized. While there is great diversity among different vertebrate nervous systems, they all share a basic structure: a CNS that contains a brain and spinal cord and a PNS made up of peripheral sensory and motor nerves. One interesting difference between the nervous systems of invertebrates and vertebrates is that the nerve cords of many invertebrates are located ventrally whereas the vertebrate spinal cords are located dorsally. There is debate among evolutionary biologists as to whether these different nervous system plans evolved separately or whether the invertebrate body plan arrangement somehow “flipped” during the evolution of vertebrates.
Link to Learning
Watch this video of biologist Mark Kirschner discussing the “flipping” phenomenon of vertebrate evolution.
The nervous system is made up of neurons, specialized cells that can receive and transmit chemical or electrical signals, and glia, cells that provide support functions for the neurons by playing an information processing role that is complementary to neurons. A neuron can be compared to an electrical wire—it transmits a signal from one place to another. Glia can be compared to the workers at the electric company who make sure wires go to the right places, maintain the wires, and take down wires that are broken. Although glia have been compared to workers, recent evidence suggests that also usurp some of the signaling functions of neurons.
There is great diversity in the types of neurons and glia that are present in different parts of the nervous system. There are four major types of neurons, and they share several important cellular components.
Neurons
The nervous system of the common laboratory fly, Drosophila melanogaster, contains around 100,000 neurons, the same number as a lobster. This number compares to 75 million in the mouse and 300 million in the octopus. A human brain contains around 86 billion neurons. Despite these very different numbers, the nervous systems of these animals control many of the same behaviors—from basic reflexes to more complicated behaviors like finding food and courting mates. The ability of neurons to communicate with each other as well as with other types of cells underlies all of these behaviors.
Most neurons share the same cellular components. But neurons are also highly specialized—different types of neurons have different sizes and shapes that relate to their functional roles.
Parts of a Neuron
Like other cells, each neuron has a cell body (or soma) that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components. Neurons also contain unique structures, illustrated in Figure \(2\) for receiving and sending the electrical signals that make neuronal communication possible. Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses. Although some neurons do not have any dendrites, some types of neurons have multiple dendrites. Dendrites can have small protrusions called dendritic spines, which further increase surface area for possible synaptic connections.
Once a signal is received by the dendrite, it then travels passively to the cell body. The cell body contains a specialized structure, the axon hillock that integrates signals from multiple synapses and serves as a junction between the cell body and an axon. An axon is a tube-like structure that propagates the integrated signal to specialized endings called axon terminals. These terminals in turn synapse on other neurons, muscle, or target organs. Chemicals released at axon terminals allow signals to be communicated to these other cells. Neurons usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons. Some axons are covered with myelin, which acts as an insulator to minimize dissipation of the electrical signal as it travels down the axon, greatly increasing the speed on conduction. This insulation is important as the axon from a human motor neuron can be as long as a meter—from the base of the spine to the toes. The myelin sheath is not actually part of the neuron. Myelin is produced by glial cells. Along the axon there are periodic gaps in the myelin sheath. These gaps are called nodes of Ranvier and are sites where the signal is “recharged” as it travels along the axon.
It is important to note that a single neuron does not act alone—neuronal communication depends on the connections that neurons make with one another (as well as with other cells, like muscle cells). Dendrites from a single neuron may receive synaptic contact from many other neurons. For example, dendrites from a Purkinje cell in the cerebellum are thought to receive contact from as many as 200,000 other neurons.
Art Connection
Which of the following statements is false?
1. The soma is the cell body of a nerve cell.
2. Myelin sheath provides an insulating layer to the dendrites.
3. Axons carry the signal from the soma to the target.
4. Dendrites carry the signal to the soma.
Types of Neurons
There are different types of neurons, and the functional role of a given neuron is intimately dependent on its structure. There is an amazing diversity of neuron shapes and sizes found in different parts of the nervous system (and across species), as illustrated by the neurons shown in Figure \(3\).
While there are many defined neuron cell subtypes, neurons are broadly divided into four basic types: unipolar, bipolar, multipolar, and pseudounipolar. Figure \(4\) illustrates these four basic neuron types. Unipolar neurons have only one structure that extends away from the soma. These neurons are not found in vertebrates but are found in insects where they stimulate muscles or glands. A bipolar neuron has one axon and one dendrite extending from the soma. An example of a bipolar neuron is a retinal bipolar cell, which receives signals from photoreceptor cells that are sensitive to light and transmits these signals to ganglion cells that carry the signal to the brain. Multipolar neurons are the most common type of neuron. Each multipolar neuron contains one axon and multiple dendrites. Multipolar neurons can be found in the central nervous system (brain and spinal cord). An example of a multipolar neuron is a Purkinje cell in the cerebellum, which has many branching dendrites but only one axon. Pseudounipolar cells share characteristics with both unipolar and bipolar cells. A pseudounipolar cell has a single process that extends from the soma, like a unipolar cell, but this process later branches into two distinct structures, like a bipolar cell. Most sensory neurons are pseudounipolar and have an axon that branches into two extensions: one connected to dendrites that receive sensory information and another that transmits this information to the spinal cord.
Everyday Connection: Neurogenesis
At one time, scientists believed that people were born with all the neurons they would ever have. Research performed during the last few decades indicates that neurogenesis, the birth of new neurons, continues into adulthood. Neurogenesis was first discovered in songbirds that produce new neurons while learning songs. For mammals, new neurons also play an important role in learning: about 1000 new neurons develop in the hippocampus (a brain structure involved in learning and memory) each day. While most of the new neurons will die, researchers found that an increase in the number of surviving new neurons in the hippocampus correlated with how well rats learned a new task. Interestingly, both exercise and some antidepressant medications also promote neurogenesis in the hippocampus. Stress has the opposite effect. While neurogenesis is quite limited compared to regeneration in other tissues, research in this area may lead to new treatments for disorders such as Alzheimer’s, stroke, and epilepsy.
How do scientists identify new neurons? A researcher can inject a compound called bromodeoxyuridine (BrdU) into the brain of an animal. While all cells will be exposed to BrdU, BrdU will only be incorporated into the DNA of newly generated cells that are in S phase. A technique called immunohistochemistry can be used to attach a fluorescent label to the incorporated BrdU, and a researcher can use fluorescent microscopy to visualize the presence of BrdU, and thus new neurons, in brain tissue. Figure \(5\) is a micrograph which shows fluorescently labeled neurons in the hippocampus of a rat.
Link to Learning
This site contains more information about neurogenesis, including an interactive laboratory simulation and a video that explains how BrdU labels new cells.
Glia
While glia are often thought of as the supporting cast of the nervous system, the number of glial cells in the brain actually outnumbers the number of neurons by a factor of ten. Neurons would be unable to function without the vital roles that are fulfilled by these glial cells. Glia guide developing neurons to their destinations, buffer ions and chemicals that would otherwise harm neurons, and provide myelin sheaths around axons. Scientists have recently discovered that they also play a role in responding to nerve activity and modulating communication between nerve cells. When glia do not function properly, the result can be disastrous—most brain tumors are caused by mutations in glia.
Types of Glia
There are several different types of glia with different functions, two of which are shown in Figure \(6\). Astrocytes, shown in Figure \(7\) make contact with both capillaries and neurons in the CNS. They provide nutrients and other substances to neurons, regulate the concentrations of ions and chemicals in the extracellular fluid, and provide structural support for synapses. Astrocytes also form the blood-brain barrier—a structure that blocks entrance of toxic substances into the brain. Astrocytes, in particular, have been shown through calcium imaging experiments to become active in response to nerve activity, transmit calcium waves between astrocytes, and modulate the activity of surrounding synapses.
Satellite glia provide nutrients and structural support for neurons in the PNS. Microglia scavenge and degrade dead cells and protect the brain from invading microorganisms. Oligodendrocytes, shown in Figure \(7\) form myelin sheaths around axons in the CNS. One axon can be myelinated by several oligodendrocytes, and one oligodendrocyte can provide myelin for multiple neurons. This is distinctive from the PNS where a single Schwann cell provides myelin for only one axon as the entire Schwann cell surrounds the axon. Radial glia serve as scaffolds for developing neurons as they migrate to their end destinations. Ependymal cells line fluid-filled ventricles of the brain and the central canal of the spinal cord. They are involved in the production of cerebrospinal fluid, which serves as a cushion for the brain, moves the fluid between the spinal cord and the brain, and is a component for the choroid plexus.
Summary
The nervous system is made up of neurons and glia. Neurons are specialized cells that are capable of sending electrical as well as chemical signals. Most neurons contain dendrites, which receive these signals, and axons that send signals to other neurons or tissues. There are four main types of neurons: unipolar, bipolar, multipolar, and pseudounipolar neurons. Glia are non-neuronal cells in the nervous system that support neuronal development and signaling. There are several types of glia that serve different functions.
Art Connections
Figure \(2\): Which of the following statements is false?
1. The soma is the cell body of a nerve cell.
2. Myelin sheath provides an insulating layer to the dendrites.
3. Axons carry the signal from the soma to the target.
4. Dendrites carry the signal to the soma.
Answer
B
Glossary
astrocyte
glial cell in the central nervous system that provide nutrients, extracellular buffering, and structural support for neurons; also makes up the blood-brain barrier
axon
tube-like structure that propagates a signal from a neuron’s cell body to axon terminals
axon hillock
electrically sensitive structure on the cell body of a neuron that integrates signals from multiple neuronal connections
axon terminal
structure on the end of an axon that can form a synapse with another neuron
dendrite
structure that extends away from the cell body to receive messages from other neurons
ependymal
cell that lines fluid-filled ventricles of the brain and the central canal of the spinal cord; involved in production of cerebrospinal fluid
glia
(also, glial cells) cells that provide support functions for neurons
microglia
glia that scavenge and degrade dead cells and protect the brain from invading microorganisms
myelin
fatty substance produced by glia that insulates axons
neuron
specialized cell that can receive and transmit electrical and chemical signals
nodes of Ranvier
gaps in the myelin sheath where the signal is recharged
oligodendrocyte
glial cell that myelinates central nervous system neuron axons
radial glia
glia that serve as scaffolds for developing neurons as they migrate to their final destinations
satellite glia
glial cell that provides nutrients and structural support for neurons in the peripheral nervous system
Schwann cell
glial cell that creates myelin sheath around a peripheral nervous system neuron axon
synapse
junction between two neurons where neuronal signals are communicated | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/41%3A_The_Animal_Body_and_Principles_of_Regulation/41.05%3A_Nerve_Tissue.txt |
Skills to Develop
• Define homeostasis
• Describe the factors affecting homeostasis
• Discuss positive and negative feedback mechanisms used in homeostasis
• Describe thermoregulation of endothermic and ectothermic animals
Animal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis (“steady state”). These changes might be in the level of glucose or calcium in blood or in external temperatures. Homeostasis means to maintain dynamic equilibrium in the body. It is dynamic because it is constantly adjusting to the changes that the body’s systems encounter. It is equilibrium because body functions are kept within specific ranges. Even an animal that is apparently inactive is maintaining this homeostatic equilibrium.
Homeostatic Process
The goal of homeostasis is the maintenance of equilibrium around a point or value called a set point. While there are normal fluctuations from the set point, the body’s systems will usually attempt to go back to this point. A change in the internal or external environment is called a stimulus and is detected by a receptor; the response of the system is to adjust the deviation parameter toward the set point. For instance, if the body becomes too warm, adjustments are made to cool the animal. If the blood’s glucose rises after a meal, adjustments are made to lower the blood glucose level by getting the nutrient into tissues that need it or to store it for later use.
Control of Homeostasis
When a change occurs in an animal’s environment, an adjustment must be made. The receptor senses the change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn generates a response that is signaled to an effector. The effector is a muscle (that contracts or relaxes) or a gland that secretes. Homeostatsis is maintained by negative feedback loops. Positive feedback loops actually push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine system of mammals.
Negative Feedback Mechanisms
Any homeostatic process that changes the direction of the stimulus is a negative feedback loop. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is too low, the body does something to make it go up. Hence the term negative feedback. An example is animal maintenance of blood glucose levels. When an animal has eaten, blood glucose levels rise. This is sensed by the nervous system. Specialized cells in the pancreas sense this, and the hormone insulin is released by the endocrine system. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system, as illustrated in Figure \(1\). However, if an animal has not eaten and blood glucose levels decrease, this is sensed in another group of cells in the pancreas, and the hormone glucagon is released causing glucose levels to increase. This is still a negative feedback loop, but not in the direction expected by the use of the term “negative.” Another example of an increase as a result of the feedback loop is the control of blood calcium. If calcium levels decrease, specialized cells in the parathyroid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys and, possibly, the breakdown of bone in order to liberate calcium. The effects of PTH are to raise blood levels of the element. Negative feedback loops are the predominant mechanism used in homeostasis.
Positive Feedback Loop
A positive feedback loop maintains the direction of the stimulus, possibly accelerating it. Few examples of positive feedback loops exist in animal bodies, but one is found in the cascade of chemical reactions that result in blood clotting, or coagulation. As one clotting factor is activated, it activates the next factor in sequence until a fibrin clot is achieved. The direction is maintained, not changed, so this is positive feedback. Another example of positive feedback is uterine contractions during childbirth, as illustrated in Figure \(2\). The hormone oxytocin, made by the endocrine system, stimulates the contraction of the uterus. This produces pain sensed by the nervous system. Instead of lowering the oxytocin and causing the pain to subside, more oxytocin is produced until the contractions are powerful enough to produce childbirth.
Exercise
State whether each of the following processes is regulated by a positive feedback loop or a negative feedback loop.
1. A person feels satiated after eating a large meal.
2. The blood has plenty of red blood cells. As a result, erythropoietin, a hormone that stimulates the production of new red blood cells, is no longer released from the kidney.
Answer
Both processes are the result of negative feedback loops. Negative feedback loops, which tend to keep a system at equilibrium, are more common than positive feedback loops.
Set Point
It is possible to adjust a system’s set point. When this happens, the feedback loop works to maintain the new setting. An example of this is blood pressure: over time, the normal or set point for blood pressure can increase as a result of continued increases in blood pressure. The body no longer recognizes the elevation as abnormal and no attempt is made to return to the lower set point. The result is the maintenance of an elevated blood pressure that can have harmful effects on the body. Medication can lower blood pressure and lower the set point in the system to a more healthy level. This is called a process of alteration of the set point in a feedback loop.
Changes can be made in a group of body organ systems in order to maintain a set point in another system. This is called acclimatization. This occurs, for instance, when an animal migrates to a higher altitude than it is accustomed to. In order to adjust to the lower oxygen levels at the new altitude, the body increases the number of red blood cells circulating in the blood to ensure adequate oxygen delivery to the tissues. Another example of acclimatization is animals that have seasonal changes in their coats: a heavier coat in the winter ensures adequate heat retention, and a light coat in summer assists in keeping body temperature from rising to harmful levels.
Link to Learning
Feedback mechanisms can be understood in terms of driving a race car along a track: watch a short video lesson on positive and negative feedback loops.
Homeostasis: Thermoregulation
Body temperature affects body activities. Generally, as body temperature rises, enzyme activity rises as well. For every ten degree centigrade rise in temperature, enzyme activity doubles, up to a point. Body proteins, including enzymes, begin to denature and lose their function with high heat (around 50oC for mammals). Enzyme activity will decrease by half for every ten degree centigrade drop in temperature, to the point of freezing, with a few exceptions. Some fish can withstand freezing solid and return to normal with thawing.
Link to Learning
Watch this Discovery Channel video on thermoregulation to see illustrations of this process in a variety of animals.
Endotherms and Ectotherms
Animals can be divided into two groups: some maintain a constant body temperature in the face of differing environmental temperatures, while others have a body temperature that is the same as their environment and thus varies with the environment. Animals that do not control their body temperature are ectotherms. This group has been called cold-blooded, but the term may not apply to an animal in the desert with a very warm body temperature. In contrast to ectotherms, which rely on external temperatures to set their body temperatures, poikilotherms are animals with constantly varying internal temperatures. An animal that maintains a constant body temperature in the face of environmental changes is called a homeotherm. Endotherms are animals that rely on internal sources for body temperature but which can exhibit extremes in temperature. These animals are able to maintain a level of activity at cooler temperature, which an ectotherm cannot due to differing enzyme levels of activity.
Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction (Figure \(3\)). Radiation is the emission of electromagnetic “heat” waves. Heat comes from the sun in this manner and radiates from dry skin the same way. Heat can be removed with liquid from a surface during evaporation. This occurs when a mammal sweats. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat will be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock.
Heat Conservation and Dissipation
Animals conserve or dissipate heat in a variety of ways. In certain climates, endothermic animals have some form of insulation, such as fur, fat, feathers, or some combination thereof. Animals with thick fur or feathers create an insulating layer of air between their skin and internal organs. Polar bears and seals live and swim in a subfreezing environment and yet maintain a constant, warm, body temperature. The arctic fox, for example, uses its fluffy tail as extra insulation when it curls up to sleep in cold weather. Mammals have a residual effect from shivering and increased muscle activity: arrector pili muscles cause “goose bumps,” causing small hairs to stand up when the individual is cold; this has the intended effect of increasing body temperature. Mammals use layers of fat to achieve the same end. Loss of significant amounts of body fat will compromise an individual’s ability to conserve heat.
Endotherms use their circulatory systems to help maintain body temperature. Vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body. Vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, and conserving heat. Some animals have adaptions to their circulatory system that enable them to transfer heat from arteries to veins, warming blood returning to the heart. This is called a countercurrent heat exchange; it prevents the cold venous blood from cooling the heart and other internal organs. This adaption can be shut down in some animals to prevent overheating the internal organs. The countercurrent adaption is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds. In contrast, similar adaptations can help cool endotherms when needed, such as dolphin flukes and elephant ears.
Some ectothermic animals use changes in their behavior to help regulate body temperature. For example, a desert ectothermic animal may simply seek cooler areas during the hottest part of the day in the desert to keep from getting too warm. The same animals may climb onto rocks to capture heat during a cold desert night. Some animals seek water to aid evaporation in cooling them, as seen with reptiles. Other ectotherms use group activity such as the activity of bees to warm a hive to survive winter.
Many animals, especially mammals, use metabolic waste heat as a heat source. When muscles are contracted, most of the energy from the ATP used in muscle actions is wasted energy that translates into heat. Severe cold elicits a shivering reflex that generates heat for the body. Many species also have a type of adipose tissue called brown fat that specializes in generating heat.
Neural Control of Thermoregulation
The nervous system is important to thermoregulation, as illustrated in Figure \(4\). The processes of homeostasis and temperature control are centered in the hypothalamus of the advanced animal brain.
Exercise
When bacteria are destroyed by leuckocytes, pyrogens are released into the blood. Pyrogens reset the body’s thermostat to a higher temperature, resulting in fever. How might pyrogens cause the body temperature to rise?
Answer
Pyrogens increase body temperature by causing the blood vessels to constrict, inducing shivering, and stopping sweat glands from secreting fluid.
The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation and sweating when the body is too warm, or vasoconstriction and shivering when the body is too cold. It responds to chemicals from the body. When a bacterium is destroyed by phagocytic leukocytes, chemicals called endogenous pyrogens are released into the blood. These pyrogens circulate to the hypothalamus and reset the thermostat. This allows the body’s temperature to increase in what is commonly called a fever. An increase in body temperature causes iron to be conserved, which reduces a nutrient needed by bacteria. An increase in body heat also increases the activity of the animal’s enzymes and protective cells while inhibiting the enzymes and activity of the invading microorganisms. Finally, heat itself may also kill the pathogen. A fever that was once thought to be a complication of an infection is now understood to be a normal defense mechanism.
Summary
Homeostasis is a dynamic equilibrium that is maintained in body tissues and organs. It is dynamic because it is constantly adjusting to the changes that the systems encounter. It is in equilibrium because body functions are kept within a normal range, with some fluctuations around a set point for the processes.
Glossary
acclimatization
alteration in a body system in response to environmental change
alteration
change of the set point in a homeostatic system
homeostasis
dynamic equilibrium maintaining appropriate body functions
negative feedback loop
feedback to a control mechanism that increases or decreases a stimulus instead of maintaining it
positive feedback loop
feedback to a control mechanism that continues the direction of a stimulus
set point
midpoint or target point in homeostasis
thermoregulation
regulation of body temperature | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/41%3A_The_Animal_Body_and_Principles_of_Regulation/41.07%3A_Homeostasis.txt |
Skills to Develop
• Describe the various types of body plans that occur in animals
• Describe limits on animal size and shape
• Relate bioenergetics to body size, levels of activity, and the environment
Animals vary in form and function. From a sponge to a worm to a goat, an organism has a distinct body plan that limits its size and shape. Animals’ bodies are also designed to interact with their environments, whether in the deep sea, a rainforest canopy, or the desert. Therefore, a large amount of information about the structure of an organism's body (anatomy) and the function of its cells, tissues and organs (physiology) can be learned by studying that organism's environment.
Body Plans
Animal body plans follow set patterns related to symmetry. They are asymmetrical, radial, or bilateral in form as illustrated in Figure \(1\). Asymmetrical animals are animals with no pattern or symmetry; an example of an asymmetrical animal is a sponge. Radial symmetry, as illustrated in Figure \(1\), describes when an animal has an up-and-down orientation: any plane cut along its longitudinal axis through the organism produces equal halves, but not a definite right or left side. This plan is found mostly in aquatic animals, especially organisms that attach themselves to a base, like a rock or a boat, and extract their food from the surrounding water as it flows around the organism. Bilateral symmetry is illustrated in the same figure by a goat. The goat also has an upper and lower component to it, but a plane cut from front to back separates the animal into definite right and left sides. Additional terms used when describing positions in the body are anterior (front), posterior (rear), dorsal (toward the back), and ventral (toward the stomach). Bilateral symmetry is found in both land-based and aquatic animals; it enables a high level of mobility.
Limits on Animal Size and Shape
Animals with bilateral symmetry that live in water tend to have a fusiform shape: this is a tubular shaped body that is tapered at both ends. This shape decreases the drag on the body as it moves through water and allows the animal to swim at high speeds. The table below lists the maximum speed of various animals. Certain types of sharks can swim at fifty kilometers an hour and some dolphins at 32 to 40 kilometers per hour. Land animals frequently travel faster, although the tortoise and snail are significantly slower than cheetahs. Another difference in the adaptations of aquatic and land-dwelling organisms is that aquatic organisms are constrained in shape by the forces of drag in the water since water has higher viscosity than air. On the other hand, land-dwelling organisms are constrained mainly by gravity, and drag is relatively unimportant. For example, most adaptations in birds are for gravity not for drag.
Table \(1\): Maximum Speed of Assorted Land and Marine Animals
Animal Speed (kmh) Speed (mph)
Cheetah 113 70
Quarter horse 77 48
Fox 68 42
Shortfin mako shark 50 31
Domestic house cat 48 30
Human 45 28
Dolphin 32–40 20–25
Mouse 13 8
Snail 0.05 0.03
Most animals have an exoskeleton, including insects, spiders, scorpions, horseshoe crabs, centipedes, and crustaceans. Scientists estimate that, of insects alone, there are over 30 million species on our planet. The exoskeleton is a hard covering or shell that provides benefits to the animal, such as protection against damage from predators and from water loss (for land animals); it also provides for the attachments of muscles.
As the tough and resistant outer cover of an arthropod, the exoskeleton may be constructed of a tough polymer such as chitin and is often biomineralized with materials such as calcium carbonate. This is fused to the animal’s epidermis. Ingrowths of the exoskeleton, called apodemes, function as attachment sites for muscles, similar to tendons in more advanced animals (Figure \(2\)). In order to grow, the animal must first synthesize a new exoskeleton underneath the old one and then shed or molt the original covering. This limits the animal’s ability to grow continually, and may limit the individual’s ability to mature if molting does not occur at the proper time. The thickness of the exoskeleton must be increased significantly to accommodate any increase in weight. It is estimated that a doubling of body size increases body weight by a factor of eight. The increasing thickness of the chitin necessary to support this weight limits most animals with an exoskeleton to a relatively small size. The same principles apply to endoskeletons, but they are more efficient because muscles are attached on the outside, making it easier to compensate for increased mass.
An animal with an endoskeleton has its size determined by the amount of skeletal system it needs in order to support the other tissues and the amount of muscle it needs for movement. As the body size increases, both bone and muscle mass increase. The speed achievable by the animal is a balance between its overall size and the bone and muscle that provide support and movement.
Limiting Effects of Diffusion on Size and Development
The exchange of nutrients and wastes between a cell and its watery environment occurs through the process of diffusion. All living cells are bathed in liquid, whether they are in a single-celled organism or a multicellular one. Diffusion is effective over a specific distance and limits the size that an individual cell can attain. If a cell is a single-celled microorganism, such as an amoeba, it can satisfy all of its nutrient and waste needs through diffusion. If the cell is too large, then diffusion is ineffective and the center of the cell does not receive adequate nutrients nor is it able to effectively dispel its waste.
An important concept in understanding how efficient diffusion is as a means of transport is the surface to volume ratio. Recall that any three-dimensional object has a surface area and volume; the ratio of these two quantities is the surface-to-volume ratio. Consider a cell shaped like a perfect sphere: it has a surface area of 4πr2, and a volume of (4/3)πr3. The surface-to-volume ratio of a sphere is 3/r; as the cell gets bigger, its surface to volume ratio decreases, making diffusion less efficient. The larger the size of the sphere, or animal, the less surface area for diffusion it possesses.
The solution to producing larger organisms is for them to become multicellular. Specialization occurs in complex organisms, allowing cells to become more efficient at doing fewer tasks. For example, circulatory systems bring nutrients and remove waste, while respiratory systems provide oxygen for the cells and remove carbon dioxide from them. Other organ systems have developed further specialization of cells and tissues and efficiently control body functions. Moreover, surface-to-volume ratio applies to other areas of animal development, such as the relationship between muscle mass and cross-sectional surface area in supporting skeletons, and in the relationship between muscle mass and the generation of dissipation of heat.
Animal Bioenergetics
All animals must obtain their energy from food they ingest or absorb. These nutrients are converted to adenosine triphosphate (ATP) for short-term storage and use by all cells. Some animals store energy for slightly longer times as glycogen, and others store energy for much longer times in the form of triglycerides housed in specialized adipose tissues. No energy system is one hundred percent efficient, and an animal’s metabolism produces waste energy in the form of heat. If an animal can conserve that heat and maintain a relatively constant body temperature, it is classified as a warm-blooded animal and called an endotherm. The insulation used to conserve the body heat comes in the forms of fur, fat, or feathers. The absence of insulation in ectothermic animals increases their dependence on the environment for body heat.
The amount of energy expended by an animal over a specific time is called its metabolic rate. The rate is measured variously in joules, calories, or kilocalories (1000 calories). Carbohydrates and proteins contain about 4.5 to 5 kcal/g, and fat contains about 9 kcal/g. Metabolic rate is estimated as the basal metabolic rate (BMR) in endothermic animals at rest and as the standard metabolic rate (SMR) in ectotherms. Human males have a BMR of 1600 to 1800 kcal/day, and human females have a BMR of 1300 to 1500 kcal/day. Even with insulation, endothermal animals require extensive amounts of energy to maintain a constant body temperature. An ectotherm such as an alligator has an SMR of 60 kcal/day.
Smaller endothermic animals have a greater surface area for their mass than larger ones (Figure \(3\)). Therefore, smaller animals lose heat at a faster rate than larger animals and require more energy to maintain a constant internal temperature. This results in a smaller endothermic animal having a higher BMR, per body weight, than a larger endothermic animal.
The more active an animal is, the more energy is needed to maintain that activity, and the higher its BMR or SMR. The average daily rate of energy consumption is about two to four times an animal’s BMR or SMR. Humans are more sedentary than most animals and have an average daily rate of only 1.5 times the BMR. The diet of an endothermic animal is determined by its BMR. For example: the type of grasses, leaves, or shrubs that an herbivore eats affects the number of calories that it takes in. The relative caloric content of herbivore foods, in descending order, is tall grasses > legumes > short grasses > forbs (any broad-leaved plant, not a grass) > subshrubs > annuals/biennials.
Animals adapt to extremes of temperature or food availability through torpor. Torpor is a process that leads to a decrease in activity and metabolism and allows animals to survive adverse conditions. Torpor can be used by animals for long periods, such as entering a state of hibernation during the winter months, in which case it enables them to maintain a reduced body temperature. During hibernation, ground squirrels can achieve an abdominal temperature of 0° C (32° F), while a bear’s internal temperature is maintained higher at about 37° C (99° F).
If torpor occurs during the summer months with high temperatures and little water, it is called estivation. Some desert animals use this to survive the harshest months of the year. Torpor can occur on a daily basis; this is seen in bats and hummingbirds. While endothermy is limited in smaller animals by surface to volume ratio, some organisms can be smaller and still be endotherms because they employ daily torpor during the part of the day that is coldest. This allows them to conserve energy during the colder parts of the day, when they consume more energy to maintain their body temperature.
Animal Body Planes and Cavities
A standing vertebrate animal can be divided by several planes. A sagittal plane divides the body into right and left portions. A midsagittal plane divides the body exactly in the middle, making two equal right and left halves. A frontal plane (also called a coronal plane) separates the front from the back. A transverse plane (or, horizontal plane) divides the animal into upper and lower portions. This is sometimes called a cross section, and, if the transverse cut is at an angle, it is called an oblique plane. Figure \(4\) illustrates these planes on a goat (a four-legged animal) and a human being.
Vertebrate animals have a number of defined body cavities, as illustrated in Figure \(5\). Two of these are major cavities that contain smaller cavities within them. The dorsal cavity contains the cranial and the vertebral (or spinal) cavities. The ventral cavity contains the thoracic cavity, which in turn contains the pleural cavity around the lungs and the pericardial cavity, which surrounds the heart. The ventral cavity also contains the abdominopelvic cavity, which can be separated into the abdominal and the pelvic cavities.
Career Connections: Physical Anthropologist
Physical anthropologists study the adaption, variability, and evolution of human beings, plus their living and fossil relatives. They can work in a variety of settings, although most will have an academic appointment at a university, usually in an anthropology department or a biology, genetics, or zoology department.
Non-academic positions are available in the automotive and aerospace industries where the focus is on human size, shape, and anatomy. Research by these professionals might range from studies of how the human body reacts to car crashes to exploring how to make seats more comfortable. Other non-academic positions can be obtained in museums of natural history, anthropology, archaeology, or science and technology. These positions involve educating students from grade school through graduate school. Physical anthropologists serve as education coordinators, collection managers, writers for museum publications, and as administrators. Zoos employ these professionals, especially if they have an expertise in primate biology; they work in collection management and captive breeding programs for endangered species. Forensic science utilizes physical anthropology expertise in identifying human and animal remains, assisting in determining the cause of death, and for expert testimony in trials.
Summary
Animal bodies come in a variety of sizes and shapes. Limits on animal size and shape include impacts to their movement. Diffusion affects their size and development. Bioenergetics describes how animals use and obtain energy in relation to their body size, activity level, and environment.
Glossary
apodeme
ingrowth of an animal’s exoskeleton that functions as an attachment site for muscles
asymmetrical
describes animals with no axis of symmetry in their body pattern
basal metabolic rate (BMR)
metabolic rate at rest in endothermic animals
dorsal cavity
body cavity on the posterior or back portion of an animal; includes the cranial and vertebral cavities
ectotherm
animal incapable of maintaining a relatively constant internal body temperature
endotherm
animal capable of maintaining a relatively constant internal body temperature
estivation
torpor in response to extremely high temperatures and low water availability
frontal (coronal) plane
plane cutting through an animal separating the individual into front and back portions
fusiform
animal body shape that is tubular and tapered at both ends
hibernation
torpor over a long period of time, such as a winter
midsagittal plane
plane cutting through an animal separating the individual into even right and left sides
sagittal plane
plane cutting through an animal separating the individual into right and left sides
standard metabolic rate (SMR)
metabolic rate at rest in ectothermic animals
torpor
decrease in activity and metabolism that allows an animal to survive adverse conditions
transverse (horizontal) plane
plane cutting through an animal separating the individual into upper and lower portions
ventral cavity
body cavity on the anterior or front portion of an animal that includes the thoracic cavities and the abdominopelvic cavities
41.08: Regulating Body Temperature
Skills to Develop
• Define homeostasis
• Describe the factors affecting homeostasis
• Discuss positive and negative feedback mechanisms used in homeostasis
• Describe thermoregulation of endothermic and ectothermic animals
Animal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis (“steady state”). These changes might be in the level of glucose or calcium in blood or in external temperatures. Homeostasis means to maintain dynamic equilibrium in the body. It is dynamic because it is constantly adjusting to the changes that the body’s systems encounter. It is equilibrium because body functions are kept within specific ranges. Even an animal that is apparently inactive is maintaining this homeostatic equilibrium.
Homeostatic Process
The goal of homeostasis is the maintenance of equilibrium around a point or value called a set point. While there are normal fluctuations from the set point, the body’s systems will usually attempt to go back to this point. A change in the internal or external environment is called a stimulus and is detected by a receptor; the response of the system is to adjust the deviation parameter toward the set point. For instance, if the body becomes too warm, adjustments are made to cool the animal. If the blood’s glucose rises after a meal, adjustments are made to lower the blood glucose level by getting the nutrient into tissues that need it or to store it for later use.
Control of Homeostasis
When a change occurs in an animal’s environment, an adjustment must be made. The receptor senses the change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn generates a response that is signaled to an effector. The effector is a muscle (that contracts or relaxes) or a gland that secretes. Homeostatsis is maintained by negative feedback loops. Positive feedback loops actually push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine system of mammals.
Negative Feedback Mechanisms
Any homeostatic process that changes the direction of the stimulus is a negative feedback loop. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is too low, the body does something to make it go up. Hence the term negative feedback. An example is animal maintenance of blood glucose levels. When an animal has eaten, blood glucose levels rise. This is sensed by the nervous system. Specialized cells in the pancreas sense this, and the hormone insulin is released by the endocrine system. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system, as illustrated in Figure \(1\). However, if an animal has not eaten and blood glucose levels decrease, this is sensed in another group of cells in the pancreas, and the hormone glucagon is released causing glucose levels to increase. This is still a negative feedback loop, but not in the direction expected by the use of the term “negative.” Another example of an increase as a result of the feedback loop is the control of blood calcium. If calcium levels decrease, specialized cells in the parathyroid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys and, possibly, the breakdown of bone in order to liberate calcium. The effects of PTH are to raise blood levels of the element. Negative feedback loops are the predominant mechanism used in homeostasis.
Positive Feedback Loop
A positive feedback loop maintains the direction of the stimulus, possibly accelerating it. Few examples of positive feedback loops exist in animal bodies, but one is found in the cascade of chemical reactions that result in blood clotting, or coagulation. As one clotting factor is activated, it activates the next factor in sequence until a fibrin clot is achieved. The direction is maintained, not changed, so this is positive feedback. Another example of positive feedback is uterine contractions during childbirth, as illustrated in Figure \(2\). The hormone oxytocin, made by the endocrine system, stimulates the contraction of the uterus. This produces pain sensed by the nervous system. Instead of lowering the oxytocin and causing the pain to subside, more oxytocin is produced until the contractions are powerful enough to produce childbirth.
Exercise
State whether each of the following processes is regulated by a positive feedback loop or a negative feedback loop.
1. A person feels satiated after eating a large meal.
2. The blood has plenty of red blood cells. As a result, erythropoietin, a hormone that stimulates the production of new red blood cells, is no longer released from the kidney.
Answer
Both processes are the result of negative feedback loops. Negative feedback loops, which tend to keep a system at equilibrium, are more common than positive feedback loops.
Set Point
It is possible to adjust a system’s set point. When this happens, the feedback loop works to maintain the new setting. An example of this is blood pressure: over time, the normal or set point for blood pressure can increase as a result of continued increases in blood pressure. The body no longer recognizes the elevation as abnormal and no attempt is made to return to the lower set point. The result is the maintenance of an elevated blood pressure that can have harmful effects on the body. Medication can lower blood pressure and lower the set point in the system to a more healthy level. This is called a process of alteration of the set point in a feedback loop.
Changes can be made in a group of body organ systems in order to maintain a set point in another system. This is called acclimatization. This occurs, for instance, when an animal migrates to a higher altitude than it is accustomed to. In order to adjust to the lower oxygen levels at the new altitude, the body increases the number of red blood cells circulating in the blood to ensure adequate oxygen delivery to the tissues. Another example of acclimatization is animals that have seasonal changes in their coats: a heavier coat in the winter ensures adequate heat retention, and a light coat in summer assists in keeping body temperature from rising to harmful levels.
Link to Learning
Feedback mechanisms can be understood in terms of driving a race car along a track: watch a short video lesson on positive and negative feedback loops.
Homeostasis: Thermoregulation
Body temperature affects body activities. Generally, as body temperature rises, enzyme activity rises as well. For every ten degree centigrade rise in temperature, enzyme activity doubles, up to a point. Body proteins, including enzymes, begin to denature and lose their function with high heat (around 50oC for mammals). Enzyme activity will decrease by half for every ten degree centigrade drop in temperature, to the point of freezing, with a few exceptions. Some fish can withstand freezing solid and return to normal with thawing.
Link to Learning
Watch this Discovery Channel video on thermoregulation to see illustrations of this process in a variety of animals.
Endotherms and Ectotherms
Animals can be divided into two groups: some maintain a constant body temperature in the face of differing environmental temperatures, while others have a body temperature that is the same as their environment and thus varies with the environment. Animals that do not control their body temperature are ectotherms. This group has been called cold-blooded, but the term may not apply to an animal in the desert with a very warm body temperature. In contrast to ectotherms, which rely on external temperatures to set their body temperatures, poikilotherms are animals with constantly varying internal temperatures. An animal that maintains a constant body temperature in the face of environmental changes is called a homeotherm. Endotherms are animals that rely on internal sources for body temperature but which can exhibit extremes in temperature. These animals are able to maintain a level of activity at cooler temperature, which an ectotherm cannot due to differing enzyme levels of activity.
Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction (Figure \(3\)). Radiation is the emission of electromagnetic “heat” waves. Heat comes from the sun in this manner and radiates from dry skin the same way. Heat can be removed with liquid from a surface during evaporation. This occurs when a mammal sweats. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat will be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock.
Heat Conservation and Dissipation
Animals conserve or dissipate heat in a variety of ways. In certain climates, endothermic animals have some form of insulation, such as fur, fat, feathers, or some combination thereof. Animals with thick fur or feathers create an insulating layer of air between their skin and internal organs. Polar bears and seals live and swim in a subfreezing environment and yet maintain a constant, warm, body temperature. The arctic fox, for example, uses its fluffy tail as extra insulation when it curls up to sleep in cold weather. Mammals have a residual effect from shivering and increased muscle activity: arrector pili muscles cause “goose bumps,” causing small hairs to stand up when the individual is cold; this has the intended effect of increasing body temperature. Mammals use layers of fat to achieve the same end. Loss of significant amounts of body fat will compromise an individual’s ability to conserve heat.
Endotherms use their circulatory systems to help maintain body temperature. Vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body. Vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, and conserving heat. Some animals have adaptions to their circulatory system that enable them to transfer heat from arteries to veins, warming blood returning to the heart. This is called a countercurrent heat exchange; it prevents the cold venous blood from cooling the heart and other internal organs. This adaption can be shut down in some animals to prevent overheating the internal organs. The countercurrent adaption is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds. In contrast, similar adaptations can help cool endotherms when needed, such as dolphin flukes and elephant ears.
Some ectothermic animals use changes in their behavior to help regulate body temperature. For example, a desert ectothermic animal may simply seek cooler areas during the hottest part of the day in the desert to keep from getting too warm. The same animals may climb onto rocks to capture heat during a cold desert night. Some animals seek water to aid evaporation in cooling them, as seen with reptiles. Other ectotherms use group activity such as the activity of bees to warm a hive to survive winter.
Many animals, especially mammals, use metabolic waste heat as a heat source. When muscles are contracted, most of the energy from the ATP used in muscle actions is wasted energy that translates into heat. Severe cold elicits a shivering reflex that generates heat for the body. Many species also have a type of adipose tissue called brown fat that specializes in generating heat.
Neural Control of Thermoregulation
The nervous system is important to thermoregulation, as illustrated in Figure \(4\). The processes of homeostasis and temperature control are centered in the hypothalamus of the advanced animal brain.
Exercise
When bacteria are destroyed by leuckocytes, pyrogens are released into the blood. Pyrogens reset the body’s thermostat to a higher temperature, resulting in fever. How might pyrogens cause the body temperature to rise?
Answer
Pyrogens increase body temperature by causing the blood vessels to constrict, inducing shivering, and stopping sweat glands from secreting fluid.
The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation and sweating when the body is too warm, or vasoconstriction and shivering when the body is too cold. It responds to chemicals from the body. When a bacterium is destroyed by phagocytic leukocytes, chemicals called endogenous pyrogens are released into the blood. These pyrogens circulate to the hypothalamus and reset the thermostat. This allows the body’s temperature to increase in what is commonly called a fever. An increase in body temperature causes iron to be conserved, which reduces a nutrient needed by bacteria. An increase in body heat also increases the activity of the animal’s enzymes and protective cells while inhibiting the enzymes and activity of the invading microorganisms. Finally, heat itself may also kill the pathogen. A fever that was once thought to be a complication of an infection is now understood to be a normal defense mechanism.
Summary
Homeostasis is a dynamic equilibrium that is maintained in body tissues and organs. It is dynamic because it is constantly adjusting to the changes that the systems encounter. It is in equilibrium because body functions are kept within a normal range, with some fluctuations around a set point for the processes.
Glossary
acclimatization
alteration in a body system in response to environmental change
alteration
change of the set point in a homeostatic system
homeostasis
dynamic equilibrium maintaining appropriate body functions
negative feedback loop
feedback to a control mechanism that increases or decreases a stimulus instead of maintaining it
positive feedback loop
feedback to a control mechanism that continues the direction of a stimulus
set point
midpoint or target point in homeostasis
thermoregulation
regulation of body temperature | textbooks/bio/Introductory_and_General_Biology/Map%3A_Raven_Biology_12th_Edition/41%3A_The_Animal_Body_and_Principles_of_Regulation/41.08%3A_Regulating_Body_Temperature/41.8.01%3A_Homeostasis.txt |
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